Show underlying errors on fetch failure (#45714)

- unwrap/flatten nested exceptions
- improve tests
- unify curl lookup

9 files changed, 198 insertions, 240 deletions

diff --git a/lib/spack/spack/fetch_strategy.py b/lib/spack/spack/fetch_strategy.py
index 4aa7f339de..4bbc143fc9 100644
--- a/lib/spack/spack/fetch_strategy.py
+++ b/lib/spack/spack/fetch_strategy.py
@@ -30,6 +30,7 @@ import re
 import shutil
 import urllib.error
 import urllib.parse
+import urllib.request
 from pathlib import PurePath
 from typing import List, Optional
 
@@ -273,10 +274,7 @@ class URLFetchStrategy(FetchStrategy):
     @property
     def curl(self):
         if not self._curl:
-            try:
-                self._curl = which("curl", required=True)
-            except CommandNotFoundError as exc:
-                tty.error(str(exc))
+            self._curl = web_util.require_curl()
         return self._curl
 
     def source_id(self):
@@ -297,27 +295,23 @@ class URLFetchStrategy(FetchStrategy):
     @_needs_stage
     def fetch(self):
         if self.archive_file:
-            tty.debug("Already downloaded {0}".format(self.archive_file))
+            tty.debug(f"Already downloaded {self.archive_file}")
             return
 
-        url = None
-        errors = []
+        errors: List[Exception] = []
         for url in self.candidate_urls:
-            if not web_util.url_exists(url):
-                tty.debug("URL does not exist: " + url)
-                continue
-
             try:
                 self._fetch_from_url(url)
                 break
             except FailedDownloadError as e:
-                errors.append(str(e))
-
-        for msg in errors:
-            tty.debug(msg)
+                errors.extend(e.exceptions)
+        else:
+            raise FailedDownloadError(*errors)
 
         if not self.archive_file:
-            raise FailedDownloadError(url)
+            raise FailedDownloadError(
+                RuntimeError(f"Missing archive {self.archive_file} after fetching")
+            )
 
     def _fetch_from_url(self, url):
         if spack.config.get("config:url_fetch_method") == "curl":
@@ -336,19 +330,20 @@ class URLFetchStrategy(FetchStrategy):
     @_needs_stage
     def _fetch_urllib(self, url):
         save_file = self.stage.save_filename
-        tty.msg("Fetching {0}".format(url))
 
-        # Run urllib but grab the mime type from the http headers
+        request = urllib.request.Request(url, headers={"User-Agent": web_util.SPACK_USER_AGENT})
+
         try:
-            url, headers, response = web_util.read_from_url(url)
-        except web_util.SpackWebError as e:
+            response = web_util.urlopen(request)
+        except (TimeoutError, urllib.error.URLError) as e:
             # clean up archive on failure.
             if self.archive_file:
                 os.remove(self.archive_file)
             if os.path.lexists(save_file):
                 os.remove(save_file)
-            msg = "urllib failed to fetch with error {0}".format(e)
-            raise FailedDownloadError(url, msg)
+            raise FailedDownloadError(e) from e
+
+        tty.msg(f"Fetching {url}")
 
         if os.path.lexists(save_file):
             os.remove(save_file)
@@ -356,7 +351,7 @@ class URLFetchStrategy(FetchStrategy):
         with open(save_file, "wb") as _open_file:
             shutil.copyfileobj(response, _open_file)
 
-        self._check_headers(str(headers))
+        self._check_headers(str(response.headers))
 
     @_needs_stage
     def _fetch_curl(self, url):
@@ -365,7 +360,7 @@ class URLFetchStrategy(FetchStrategy):
         if self.stage.save_filename:
             save_file = self.stage.save_filename
             partial_file = self.stage.save_filename + ".part"
-        tty.msg("Fetching {0}".format(url))
+        tty.msg(f"Fetching {url}")
         if partial_file:
             save_args = [
                 "-C",
@@ -405,8 +400,8 @@ class URLFetchStrategy(FetchStrategy):
 
             try:
                 web_util.check_curl_code(curl.returncode)
-            except spack.error.FetchError as err:
-                raise spack.fetch_strategy.FailedDownloadError(url, str(err))
+            except spack.error.FetchError as e:
+                raise FailedDownloadError(e) from e
 
         self._check_headers(headers)
 
@@ -560,7 +555,7 @@ class OCIRegistryFetchStrategy(URLFetchStrategy):
                 os.remove(self.archive_file)
             if os.path.lexists(file):
                 os.remove(file)
-            raise FailedDownloadError(self.url, f"Failed to fetch {self.url}: {e}") from e
+            raise FailedDownloadError(e) from e
 
         if os.path.lexists(file):
             os.remove(file)
@@ -1312,35 +1307,41 @@ class S3FetchStrategy(URLFetchStrategy):
     @_needs_stage
     def fetch(self):
         if self.archive_file:
-            tty.debug("Already downloaded {0}".format(self.archive_file))
+            tty.debug(f"Already downloaded {self.archive_file}")
             return
 
         parsed_url = urllib.parse.urlparse(self.url)
         if parsed_url.scheme != "s3":
             raise spack.error.FetchError("S3FetchStrategy can only fetch from s3:// urls.")
 
-        tty.debug("Fetching {0}".format(self.url))
-
         basename = os.path.basename(parsed_url.path)
+        request = urllib.request.Request(
+            self.url, headers={"User-Agent": web_util.SPACK_USER_AGENT}
+        )
 
         with working_dir(self.stage.path):
-            _, headers, stream = web_util.read_from_url(self.url)
+            try:
+                response = web_util.urlopen(request)
+            except (TimeoutError, urllib.error.URLError) as e:
+                raise FailedDownloadError(e) from e
+
+            tty.debug(f"Fetching {self.url}")
 
             with open(basename, "wb") as f:
-                shutil.copyfileobj(stream, f)
+                shutil.copyfileobj(response, f)
 
-            content_type = web_util.get_header(headers, "Content-type")
+            content_type = web_util.get_header(response.headers, "Content-type")
 
         if content_type == "text/html":
             warn_content_type_mismatch(self.archive_file or "the archive")
 
         if self.stage.save_filename:
-            llnl.util.filesystem.rename(
-                os.path.join(self.stage.path, basename), self.stage.save_filename
-            )
+            fs.rename(os.path.join(self.stage.path, basename), self.stage.save_filename)
 
         if not self.archive_file:
-            raise FailedDownloadError(self.url)
+            raise FailedDownloadError(
+                RuntimeError(f"Missing archive {self.archive_file} after fetching")
+            )
 
 
 @fetcher
@@ -1366,17 +1367,23 @@ class GCSFetchStrategy(URLFetchStrategy):
         if parsed_url.scheme != "gs":
             raise spack.error.FetchError("GCSFetchStrategy can only fetch from gs:// urls.")
 
-        tty.debug("Fetching {0}".format(self.url))
-
         basename = os.path.basename(parsed_url.path)
+        request = urllib.request.Request(
+            self.url, headers={"User-Agent": web_util.SPACK_USER_AGENT}
+        )
 
         with working_dir(self.stage.path):
-            _, headers, stream = web_util.read_from_url(self.url)
+            try:
+                response = web_util.urlopen(request)
+            except (TimeoutError, urllib.error.URLError) as e:
+                raise FailedDownloadError(e) from e
+
+            tty.debug(f"Fetching {self.url}")
 
             with open(basename, "wb") as f:
-                shutil.copyfileobj(stream, f)
+                shutil.copyfileobj(response, f)
 
-            content_type = web_util.get_header(headers, "Content-type")
+            content_type = web_util.get_header(response.headers, "Content-type")
 
         if content_type == "text/html":
             warn_content_type_mismatch(self.archive_file or "the archive")
@@ -1385,7 +1392,9 @@ class GCSFetchStrategy(URLFetchStrategy):
             os.rename(os.path.join(self.stage.path, basename), self.stage.save_filename)
 
         if not self.archive_file:
-            raise FailedDownloadError(self.url)
+            raise FailedDownloadError(
+                RuntimeError(f"Missing archive {self.archive_file} after fetching")
+            )
 
 
 @fetcher
@@ -1722,9 +1731,9 @@ class NoCacheError(spack.error.FetchError):
 class FailedDownloadError(spack.error.FetchError):
     """Raised when a download fails."""
 
-    def __init__(self, url, msg=""):
-        super().__init__("Failed to fetch file from URL: %s" % url, msg)
-        self.url = url
+    def __init__(self, *exceptions: Exception):
+        super().__init__("Failed to download")
+        self.exceptions = exceptions
 
 
 class NoArchiveFileError(spack.error.FetchError):
diff --git a/lib/spack/spack/stage.py b/lib/spack/spack/stage.py
index a635d95aeb..847c64d03f 100644
--- a/lib/spack/spack/stage.py
+++ b/lib/spack/spack/stage.py
@@ -13,7 +13,7 @@ import shutil
 import stat
 import sys
 import tempfile
-from typing import Callable, Dict, Iterable, Optional, Set
+from typing import Callable, Dict, Iterable, List, Optional, Set
 
 import llnl.string
 import llnl.util.lang
@@ -40,6 +40,7 @@ import spack.paths
 import spack.resource
 import spack.spec
 import spack.stage
+import spack.util.crypto
 import spack.util.lock
 import spack.util.path as sup
 import spack.util.pattern as pattern
@@ -534,32 +535,29 @@ class Stage(LockableStagingDir):
                 for fetcher in dynamic_fetchers:
                     yield fetcher
 
-        def print_errors(errors):
-            for msg in errors:
-                tty.debug(msg)
-
-        errors = []
+        errors: List[str] = []
         for fetcher in generate_fetchers():
             try:
                 fetcher.stage = self
                 self.fetcher = fetcher
                 self.fetcher.fetch()
                 break
-            except spack.fetch_strategy.NoCacheError:
+            except fs.NoCacheError:
                 # Don't bother reporting when something is not cached.
                 continue
+            except fs.FailedDownloadError as f:
+                errors.extend(f"{fetcher}: {e.__class__.__name__}: {e}" for e in f.exceptions)
+                continue
             except spack.error.SpackError as e:
-                errors.append("Fetching from {0} failed.".format(fetcher))
-                tty.debug(e)
+                errors.append(f"{fetcher}: {e.__class__.__name__}: {e}")
                 continue
         else:
-            print_errors(errors)
-
             self.fetcher = self.default_fetcher
-            default_msg = "All fetchers failed for {0}".format(self.name)
-            raise spack.error.FetchError(err_msg or default_msg, None)
-
-        print_errors(errors)
+            if err_msg:
+                raise spack.error.FetchError(err_msg)
+            raise spack.error.FetchError(
+                f"All fetchers failed for {self.name}", "\n".join(f"    {e}" for e in errors)
+            )
 
     def steal_source(self, dest):
         """Copy the source_path directory in its entirety to directory dest
@@ -1188,7 +1186,7 @@ def _fetch_and_checksum(url, options, keep_stage, action_fn=None):
             # Checksum the archive and add it to the list
             checksum = spack.util.crypto.checksum(hashlib.sha256, stage.archive_file)
         return checksum, None
-    except FailedDownloadError:
+    except fs.FailedDownloadError:
         return None, f"[WORKER] Failed to fetch {url}"
     except Exception as e:
         return None, f"[WORKER] Something failed on {url}, skipping.  ({e})"
@@ -1208,7 +1206,3 @@ class RestageError(StageError):
 
 class VersionFetchError(StageError):
     """Raised when we can't determine a URL to fetch a package."""
-
-
-# Keep this in namespace for convenience
-FailedDownloadError = fs.FailedDownloadError
diff --git a/lib/spack/spack/test/config.py b/lib/spack/spack/test/config.py
index 7dccdcc182..7c1c8f365b 100644
--- a/lib/spack/spack/test/config.py
+++ b/lib/spack/spack/test/config.py
@@ -18,6 +18,7 @@ from llnl.util.filesystem import join_path, touch, touchp
 import spack.config
 import spack.directory_layout
 import spack.environment as ev
+import spack.fetch_strategy
 import spack.main
 import spack.package_base
 import spack.paths
diff --git a/lib/spack/spack/test/conftest.py b/lib/spack/spack/test/conftest.py
index 8a1f887fbe..cb978b97f3 100644
--- a/lib/spack/spack/test/conftest.py
+++ b/lib/spack/spack/test/conftest.py
@@ -59,6 +59,7 @@ import spack.util.gpg
 import spack.util.parallel
 import spack.util.spack_yaml as syaml
 import spack.util.url as url_util
+import spack.util.web
 import spack.version
 from spack.fetch_strategy import URLFetchStrategy
 from spack.util.pattern import Bunch
@@ -1812,12 +1813,7 @@ def mock_curl_configs(mock_config_data, monkeypatch):
                     tty.msg("curl: (22) The requested URL returned error: 404")
                     self.returncode = 22
 
-    def mock_curl(*args):
-        return MockCurl()
-
-    monkeypatch.setattr(spack.util.web, "_curl", mock_curl)
-
-    yield
+    monkeypatch.setattr(spack.util.web, "require_curl", MockCurl)
 
 
 @pytest.fixture(scope="function")
diff --git a/lib/spack/spack/test/packaging.py b/lib/spack/spack/test/packaging.py
index aa1e50468a..e79cdd91c8 100644
--- a/lib/spack/spack/test/packaging.py
+++ b/lib/spack/spack/test/packaging.py
@@ -11,6 +11,7 @@ import os
 import pathlib
 import platform
 import shutil
+import urllib.error
 from collections import OrderedDict
 
 import pytest
@@ -21,6 +22,7 @@ from llnl.util.symlink import readlink, symlink
 import spack.binary_distribution as bindist
 import spack.cmd.buildcache as buildcache
 import spack.error
+import spack.fetch_strategy
 import spack.package_base
 import spack.repo
 import spack.store
@@ -478,7 +480,7 @@ def test_macho_make_paths():
 
 
 @pytest.fixture()
-def mock_download():
+def mock_download(monkeypatch):
     """Mock a failing download strategy."""
 
     class FailedDownloadStrategy(spack.fetch_strategy.FetchStrategy):
@@ -487,19 +489,14 @@ def mock_download():
 
         def fetch(self):
             raise spack.fetch_strategy.FailedDownloadError(
-                "<non-existent URL>", "This FetchStrategy always fails"
+                urllib.error.URLError("This FetchStrategy always fails")
             )
 
-    fetcher = FailedDownloadStrategy()
-
     @property
     def fake_fn(self):
-        return fetcher
+        return FailedDownloadStrategy()
 
-    orig_fn = spack.package_base.PackageBase.fetcher
-    spack.package_base.PackageBase.fetcher = fake_fn
-    yield
-    spack.package_base.PackageBase.fetcher = orig_fn
+    monkeypatch.setattr(spack.package_base.PackageBase, "fetcher", fake_fn)
 
 
 @pytest.mark.parametrize(
diff --git a/lib/spack/spack/test/stage.py b/lib/spack/spack/test/stage.py
index 7bb7f3753a..084d95475c 100644
--- a/lib/spack/spack/test/stage.py
+++ b/lib/spack/spack/test/stage.py
@@ -18,6 +18,7 @@ from llnl.util.filesystem import getuid, mkdirp, partition_path, touch, working_
 from llnl.util.symlink import readlink
 
 import spack.error
+import spack.fetch_strategy
 import spack.paths
 import spack.stage
 import spack.util.executable
@@ -323,17 +324,11 @@ def failing_search_fn():
     return _mock
 
 
-@pytest.fixture
-def failing_fetch_strategy():
-    """Returns a fetch strategy that fails."""
-
-    class FailingFetchStrategy(spack.fetch_strategy.FetchStrategy):
-        def fetch(self):
-            raise spack.fetch_strategy.FailedDownloadError(
-                "<non-existent URL>", "This implementation of FetchStrategy always fails"
-            )
-
-    return FailingFetchStrategy()
+class FailingFetchStrategy(spack.fetch_strategy.FetchStrategy):
+    def fetch(self):
+        raise spack.fetch_strategy.FailedDownloadError(
+            "<non-existent URL>", "This implementation of FetchStrategy always fails"
+        )
 
 
 @pytest.fixture
@@ -511,8 +506,8 @@ class TestStage:
             stage.fetch()
         check_destroy(stage, self.stage_name)
 
-    def test_no_search_mirror_only(self, failing_fetch_strategy, failing_search_fn):
-        stage = Stage(failing_fetch_strategy, name=self.stage_name, search_fn=failing_search_fn)
+    def test_no_search_mirror_only(self, failing_search_fn):
+        stage = Stage(FailingFetchStrategy(), name=self.stage_name, search_fn=failing_search_fn)
         with stage:
             try:
                 stage.fetch(mirror_only=True)
@@ -527,8 +522,8 @@ class TestStage:
             (None, "All fetchers failed"),
         ],
     )
-    def test_search_if_default_fails(self, failing_fetch_strategy, search_fn, err_msg, expected):
-        stage = Stage(failing_fetch_strategy, name=self.stage_name, search_fn=search_fn)
+    def test_search_if_default_fails(self, search_fn, err_msg, expected):
+        stage = Stage(FailingFetchStrategy(), name=self.stage_name, search_fn=search_fn)
 
         with stage:
             with pytest.raises(spack.error.FetchError, match=expected):
diff --git a/lib/spack/spack/test/url_fetch.py b/lib/spack/spack/test/url_fetch.py
index 74505b3688..96460cd9b3 100644
--- a/lib/spack/spack/test/url_fetch.py
+++ b/lib/spack/spack/test/url_fetch.py
@@ -4,8 +4,10 @@
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
 import collections
+import filecmp
 import os
 import sys
+import urllib.error
 
 import pytest
 
@@ -24,6 +26,14 @@ from spack.stage import Stage
 from spack.util.executable import which
 
 
+@pytest.fixture
+def missing_curl(monkeypatch):
+    def require_curl():
+        raise spack.error.FetchError("curl is required but not found")
+
+    monkeypatch.setattr(web_util, "require_curl", require_curl)
+
+
 @pytest.fixture(params=list(crypto.hashes.keys()))
 def checksum_type(request):
     return request.param
@@ -66,66 +76,62 @@ def pkg_factory():
     return factory
 
 
-@pytest.mark.parametrize("_fetch_method", ["curl", "urllib"])
-def test_urlfetchstrategy_sans_url(_fetch_method):
+def test_urlfetchstrategy_sans_url():
     """Ensure constructor with no URL fails."""
-    with spack.config.override("config:url_fetch_method", _fetch_method):
-        with pytest.raises(ValueError):
-            with fs.URLFetchStrategy(None):
-                pass
+    with pytest.raises(ValueError):
+        fs.URLFetchStrategy(None)
 
 
-@pytest.mark.parametrize("_fetch_method", ["curl", "urllib"])
-def test_urlfetchstrategy_bad_url(tmpdir, _fetch_method):
+@pytest.mark.parametrize("method", ["curl", "urllib"])
+def test_urlfetchstrategy_bad_url(tmp_path, mutable_config, method):
     """Ensure fetch with bad URL fails as expected."""
-    testpath = str(tmpdir)
-    with spack.config.override("config:url_fetch_method", _fetch_method):
-        with pytest.raises(fs.FailedDownloadError):
-            fetcher = fs.URLFetchStrategy(url="file:///does-not-exist")
-            assert fetcher is not None
+    mutable_config.set("config:url_fetch_method", method)
+    fetcher = fs.URLFetchStrategy(url=(tmp_path / "does-not-exist").as_uri())
 
-            with Stage(fetcher, path=testpath) as stage:
-                assert stage is not None
-                assert fetcher.archive_file is None
-                fetcher.fetch()
+    with Stage(fetcher, path=str(tmp_path / "stage")):
+        with pytest.raises(fs.FailedDownloadError) as exc:
+            fetcher.fetch()
 
+    assert len(exc.value.exceptions) == 1
+    exception = exc.value.exceptions[0]
 
-def test_fetch_options(tmpdir, mock_archive):
-    testpath = str(tmpdir)
+    if method == "curl":
+        assert isinstance(exception, spack.error.FetchError)
+        assert "Curl failed with error 37" in str(exception)  # FILE_COULDNT_READ_FILE
+    elif method == "urllib":
+        assert isinstance(exception, urllib.error.URLError)
+        assert isinstance(exception.reason, FileNotFoundError)
+
+
+def test_fetch_options(tmp_path, mock_archive):
     with spack.config.override("config:url_fetch_method", "curl"):
         fetcher = fs.URLFetchStrategy(
             url=mock_archive.url, fetch_options={"cookie": "True", "timeout": 10}
         )
-        assert fetcher is not None
 
-        with Stage(fetcher, path=testpath) as stage:
-            assert stage is not None
+        with Stage(fetcher, path=str(tmp_path)):
             assert fetcher.archive_file is None
             fetcher.fetch()
+            assert filecmp.cmp(fetcher.archive_file, mock_archive.archive_file)
 
 
 @pytest.mark.parametrize("_fetch_method", ["curl", "urllib"])
-def test_archive_file_errors(tmpdir, mock_archive, _fetch_method):
+def test_archive_file_errors(tmp_path, mutable_config, mock_archive, _fetch_method):
     """Ensure FetchStrategy commands may only be used as intended"""
-    testpath = str(tmpdir)
     with spack.config.override("config:url_fetch_method", _fetch_method):
         fetcher = fs.URLFetchStrategy(url=mock_archive.url)
-        assert fetcher is not None
-        with pytest.raises(fs.FailedDownloadError):
-            with Stage(fetcher, path=testpath) as stage:
-                assert stage is not None
-                assert fetcher.archive_file is None
-                with pytest.raises(fs.NoArchiveFileError):
-                    fetcher.archive(testpath)
-                with pytest.raises(fs.NoArchiveFileError):
-                    fetcher.expand()
-                with pytest.raises(fs.NoArchiveFileError):
-                    fetcher.reset()
-                stage.fetch()
-                with pytest.raises(fs.NoDigestError):
-                    fetcher.check()
-                assert fetcher.archive_file is not None
-                fetcher._fetch_from_url("file:///does-not-exist")
+        with Stage(fetcher, path=str(tmp_path)) as stage:
+            assert fetcher.archive_file is None
+            with pytest.raises(fs.NoArchiveFileError):
+                fetcher.archive(str(tmp_path))
+            with pytest.raises(fs.NoArchiveFileError):
+                fetcher.expand()
+            with pytest.raises(fs.NoArchiveFileError):
+                fetcher.reset()
+            stage.fetch()
+            with pytest.raises(fs.NoDigestError):
+                fetcher.check()
+            assert filecmp.cmp(fetcher.archive_file, mock_archive.archive_file)
 
 
 files = [(".tar.gz", "z"), (".tgz", "z")]
@@ -271,16 +277,15 @@ def test_url_with_status_bar(tmpdir, mock_archive, monkeypatch, capfd):
 
 
 @pytest.mark.parametrize("_fetch_method", ["curl", "urllib"])
-def test_url_extra_fetch(tmpdir, mock_archive, _fetch_method):
+def test_url_extra_fetch(tmp_path, mutable_config, mock_archive, _fetch_method):
     """Ensure a fetch after downloading is effectively a no-op."""
-    with spack.config.override("config:url_fetch_method", _fetch_method):
-        testpath = str(tmpdir)
-        fetcher = fs.URLFetchStrategy(mock_archive.url)
-        with Stage(fetcher, path=testpath) as stage:
-            assert fetcher.archive_file is None
-            stage.fetch()
-            assert fetcher.archive_file is not None
-            fetcher.fetch()
+    mutable_config.set("config:url_fetch_method", _fetch_method)
+    fetcher = fs.URLFetchStrategy(mock_archive.url)
+    with Stage(fetcher, path=str(tmp_path)) as stage:
+        assert fetcher.archive_file is None
+        stage.fetch()
+        assert filecmp.cmp(fetcher.archive_file, mock_archive.archive_file)
+        fetcher.fetch()
 
 
 @pytest.mark.parametrize(
@@ -316,49 +321,25 @@ def test_candidate_urls(pkg_factory, url, urls, version, expected, _fetch_method
 
 
 @pytest.mark.regression("19673")
-def test_missing_curl(tmpdir, monkeypatch):
+def test_missing_curl(tmp_path, missing_curl, mutable_config, monkeypatch):
     """Ensure a fetch involving missing curl package reports the error."""
-    err_fmt = "No such command {0}"
-
-    def _which(*args, **kwargs):
-        err_msg = err_fmt.format(args[0])
-        raise spack.util.executable.CommandNotFoundError(err_msg)
-
-    # Patching the 'which' symbol imported by fetch_strategy needed due
-    # to 'from spack.util.executable import which' in this module.
-    monkeypatch.setattr(fs, "which", _which)
-
-    testpath = str(tmpdir)
-    url = "http://github.com/spack/spack"
-    with spack.config.override("config:url_fetch_method", "curl"):
-        fetcher = fs.URLFetchStrategy(url=url)
-        assert fetcher is not None
-        with pytest.raises(TypeError, match="object is not callable"):
-            with Stage(fetcher, path=testpath) as stage:
-                out = stage.fetch()
-            assert err_fmt.format("curl") in out
+    mutable_config.set("config:url_fetch_method", "curl")
+    fetcher = fs.URLFetchStrategy(url="http://example.com/file.tar.gz")
+    with pytest.raises(spack.error.FetchError, match="curl is required but not found"):
+        with Stage(fetcher, path=str(tmp_path)) as stage:
+            stage.fetch()
 
 
-def test_url_fetch_text_without_url(tmpdir):
+def test_url_fetch_text_without_url():
     with pytest.raises(spack.error.FetchError, match="URL is required"):
         web_util.fetch_url_text(None)
 
 
-def test_url_fetch_text_curl_failures(tmpdir, monkeypatch):
+def test_url_fetch_text_curl_failures(mutable_config, missing_curl, monkeypatch):
     """Check fetch_url_text if URL's curl is missing."""
-    err_fmt = "No such command {0}"
-
-    def _which(*args, **kwargs):
-        err_msg = err_fmt.format(args[0])
-        raise spack.util.executable.CommandNotFoundError(err_msg)
-
-    # Patching the 'which' symbol imported by spack.util.web needed due
-    # to 'from spack.util.executable import which' in this module.
-    monkeypatch.setattr(spack.util.web, "which", _which)
-
-    with spack.config.override("config:url_fetch_method", "curl"):
-        with pytest.raises(spack.error.FetchError, match="Missing required curl"):
-            web_util.fetch_url_text("https://github.com/")
+    mutable_config.set("config:url_fetch_method", "curl")
+    with pytest.raises(spack.error.FetchError, match="curl is required but not found"):
+        web_util.fetch_url_text("https://example.com/")
 
 
 def test_url_check_curl_errors():
@@ -372,24 +353,14 @@ def test_url_check_curl_errors():
         web_util.check_curl_code(60)
 
 
-def test_url_missing_curl(tmpdir, monkeypatch):
+def test_url_missing_curl(mutable_config, missing_curl, monkeypatch):
     """Check url_exists failures if URL's curl is missing."""
-    err_fmt = "No such command {0}"
-
-    def _which(*args, **kwargs):
-        err_msg = err_fmt.format(args[0])
-        raise spack.util.executable.CommandNotFoundError(err_msg)
-
-    # Patching the 'which' symbol imported by spack.util.web needed due
-    # to 'from spack.util.executable import which' in this module.
-    monkeypatch.setattr(spack.util.web, "which", _which)
-
-    with spack.config.override("config:url_fetch_method", "curl"):
-        with pytest.raises(spack.error.FetchError, match="Missing required curl"):
-            web_util.url_exists("https://github.com/")
+    mutable_config.set("config:url_fetch_method", "curl")
+    with pytest.raises(spack.error.FetchError, match="curl is required but not found"):
+        web_util.url_exists("https://example.com/")
 
 
-def test_url_fetch_text_urllib_bad_returncode(tmpdir, monkeypatch):
+def test_url_fetch_text_urllib_bad_returncode(mutable_config, monkeypatch):
     class response:
         def getcode(self):
             return 404
@@ -397,19 +368,19 @@ def test_url_fetch_text_urllib_bad_returncode(tmpdir, monkeypatch):
     def _read_from_url(*args, **kwargs):
         return None, None, response()
 
-    monkeypatch.setattr(spack.util.web, "read_from_url", _read_from_url)
+    monkeypatch.setattr(web_util, "read_from_url", _read_from_url)
+    mutable_config.set("config:url_fetch_method", "urllib")
 
-    with spack.config.override("config:url_fetch_method", "urllib"):
-        with pytest.raises(spack.error.FetchError, match="failed with error code"):
-            web_util.fetch_url_text("https://github.com/")
+    with pytest.raises(spack.error.FetchError, match="failed with error code"):
+        web_util.fetch_url_text("https://example.com/")
 
 
-def test_url_fetch_text_urllib_web_error(tmpdir, monkeypatch):
+def test_url_fetch_text_urllib_web_error(mutable_config, monkeypatch):
     def _raise_web_error(*args, **kwargs):
         raise web_util.SpackWebError("bad url")
 
-    monkeypatch.setattr(spack.util.web, "read_from_url", _raise_web_error)
+    monkeypatch.setattr(web_util, "read_from_url", _raise_web_error)
+    mutable_config.set("config:url_fetch_method", "urllib")
 
-    with spack.config.override("config:url_fetch_method", "urllib"):
-        with pytest.raises(spack.error.FetchError, match="fetch failed to verify"):
-            web_util.fetch_url_text("https://github.com/")
+    with pytest.raises(spack.error.FetchError, match="fetch failed to verify"):
+        web_util.fetch_url_text("https://example.com/")
diff --git a/lib/spack/spack/test/web.py b/lib/spack/spack/test/web.py
index a2b64798d0..cf89e2e3a4 100644
--- a/lib/spack/spack/test/web.py
+++ b/lib/spack/spack/test/web.py
@@ -432,7 +432,7 @@ def test_ssl_curl_cert_file(cert_exists, tmpdir, ssl_scrubbed_env, mutable_confi
         if cert_exists:
             open(mock_cert, "w").close()
             assert os.path.isfile(mock_cert)
-        curl = spack.util.web._curl()
+        curl = spack.util.web.require_curl()
 
         # arbitrary call to query the run env
         dump_env = {}
diff --git a/lib/spack/spack/util/web.py b/lib/spack/spack/util/web.py
index b681bb4950..6b27c6ae68 100644
--- a/lib/spack/spack/util/web.py
+++ b/lib/spack/spack/util/web.py
@@ -28,10 +28,11 @@ from llnl.util.filesystem import mkdirp, rename, working_dir
 
 import spack.config
 import spack.error
+import spack.util.executable
 import spack.util.path
 import spack.util.url as url_util
 
-from .executable import CommandNotFoundError, Executable, which
+from .executable import CommandNotFoundError, Executable
 from .gcs import GCSBlob, GCSBucket, GCSHandler
 from .s3 import UrllibS3Handler, get_s3_session
 
@@ -198,7 +199,7 @@ def read_from_url(url, accept_content_type=None):
     try:
         response = urlopen(request)
     except (TimeoutError, URLError) as e:
-        raise SpackWebError(f"Download of {url.geturl()} failed: {e}")
+        raise SpackWebError(f"Download of {url.geturl()} failed: {e.__class__.__name__}: {e}")
 
     if accept_content_type:
         try:
@@ -307,45 +308,44 @@ def base_curl_fetch_args(url, timeout=0):
     return curl_args
 
 
-def check_curl_code(returncode):
+def check_curl_code(returncode: int) -> None:
     """Check standard return code failures for provided arguments.
 
     Arguments:
-        returncode (int): curl return code
+        returncode: curl return code
 
     Raises FetchError if the curl returncode indicates failure
     """
-    if returncode != 0:
-        if returncode == 22:
-            # This is a 404. Curl will print the error.
-            raise spack.error.FetchError("URL was not found!")
-
-        if returncode == 60:
-            # This is a certificate error.  Suggest spack -k
-            raise spack.error.FetchError(
-                "Curl was unable to fetch due to invalid certificate. "
-                "This is either an attack, or your cluster's SSL "
-                "configuration is bad.  If you believe your SSL "
-                "configuration is bad, you can try running spack -k, "
-                "which will not check SSL certificates."
-                "Use this at your own risk."
-            )
+    if returncode == 0:
+        return
+    elif returncode == 22:
+        # This is a 404. Curl will print the error.
+        raise spack.error.FetchError("URL was not found!")
+    elif returncode == 60:
+        # This is a certificate error.  Suggest spack -k
+        raise spack.error.FetchError(
+            "Curl was unable to fetch due to invalid certificate. "
+            "This is either an attack, or your cluster's SSL "
+            "configuration is bad.  If you believe your SSL "
+            "configuration is bad, you can try running spack -k, "
+            "which will not check SSL certificates."
+            "Use this at your own risk."
+        )
 
-        raise spack.error.FetchError("Curl failed with error {0}".format(returncode))
+    raise spack.error.FetchError(f"Curl failed with error {returncode}")
 
 
-def _curl(curl=None):
-    if not curl:
-        try:
-            curl = which("curl", required=True)
-        except CommandNotFoundError as exc:
-            tty.error(str(exc))
-            raise spack.error.FetchError("Missing required curl fetch method")
+def require_curl() -> Executable:
+    try:
+        path = spack.util.executable.which_string("curl", required=True)
+    except CommandNotFoundError as e:
+        raise spack.error.FetchError(f"curl is required but not found: {e}") from e
+    curl = spack.util.executable.Executable(path)
     set_curl_env_for_ssl_certs(curl)
     return curl
 
 
-def fetch_url_text(url, curl=None, dest_dir="."):
+def fetch_url_text(url, curl: Optional[Executable] = None, dest_dir="."):
     """Retrieves text-only URL content using the configured fetch method.
     It determines the fetch method from:
 
@@ -379,10 +379,7 @@ def fetch_url_text(url, curl=None, dest_dir="."):
     fetch_method = spack.config.get("config:url_fetch_method")
     tty.debug("Using '{0}' to fetch {1} into {2}".format(fetch_method, url, path))
     if fetch_method == "curl":
-        curl_exe = _curl(curl)
-        if not curl_exe:
-            raise spack.error.FetchError("Missing required fetch method (curl)")
-
+        curl_exe = curl or require_curl()
         curl_args = ["-O"]
         curl_args.extend(base_curl_fetch_args(url))
 
@@ -439,9 +436,7 @@ def url_exists(url, curl=None):
         "config:url_fetch_method", "urllib"
     ) == "curl" and url_result.scheme not in ("gs", "s3")
     if use_curl:
-        curl_exe = _curl(curl)
-        if not curl_exe:
-            return False
+        curl_exe = curl or require_curl()
 
         # Telling curl to fetch the first byte (-r 0-0) is supposed to be
         # portable.