3 files changed, 80 insertions, 3 deletions
diff --git a/lib/spack/spack/ci.py b/lib/spack/spack/ci.py
index bd664c664d..7237c95e69 100644
--- a/lib/spack/spack/ci.py
+++ b/lib/spack/spack/ci.py
@@ -22,6 +22,8 @@ from urllib.error import HTTPError, URLError
 from urllib.parse import urlencode
 from urllib.request import HTTPHandler, Request, build_opener
 
+import ruamel.yaml
+
 import llnl.util.filesystem as fs
 import llnl.util.tty as tty
 from llnl.util.lang import memoized
@@ -1310,8 +1312,11 @@ def generate_gitlab_ci_yaml(
         if not rebuild_everything:
             sys.exit(1)
 
-    with open(output_file, "w") as outf:
-        outf.write(syaml.dump(sorted_output, default_flow_style=True))
+    # Minimize yaml output size through use of anchors
+    syaml.anchorify(sorted_output)
+
+    with open(output_file, "w") as f:
+        ruamel.yaml.YAML().dump(sorted_output, f)
 
 
 def _url_encode_string(input_string):
diff --git a/lib/spack/spack/test/spec_yaml.py b/lib/spack/spack/test/spec_yaml.py
index 5dd854628a..df63fdf72e 100644
--- a/lib/spack/spack/test/spec_yaml.py
+++ b/lib/spack/spack/test/spec_yaml.py
@@ -13,10 +13,12 @@ import collections
 import collections.abc
 import gzip
 import inspect
+import io
 import json
 import os
 
 import pytest
+import ruamel.yaml
 
 import spack.hash_types as ht
 import spack.paths
@@ -505,3 +507,50 @@ def test_load_json_specfiles(specfile, expected_hash, reader_cls):
     # JSON or YAML file, not a list
     for edge in s2.traverse_edges():
         assert isinstance(edge.virtuals, tuple), edge
+
+
+def test_anchorify_1():
+    """Test that anchorify replaces duplicate values with references to a single instance, and
+    that that results in anchors in the output YAML."""
+    before = {"a": [1, 2, 3], "b": [1, 2, 3]}
+    after = {"a": [1, 2, 3], "b": [1, 2, 3]}
+    syaml.anchorify(after)
+    assert before == after
+    assert after["a"] is after["b"]
+
+    # Check if anchors are used
+    out = io.StringIO()
+    ruamel.yaml.YAML().dump(after, out)
+    assert (
+        out.getvalue()
+        == """\
+a: &id001
+- 1
+- 2
+- 3
+b: *id001
+"""
+    )
+
+
+def test_anchorify_2():
+    before = {"a": {"b": {"c": True}}, "d": {"b": {"c": True}}, "e": {"c": True}}
+    after = {"a": {"b": {"c": True}}, "d": {"b": {"c": True}}, "e": {"c": True}}
+    syaml.anchorify(after)
+    assert before == after
+    assert after["a"] is after["d"]
+    assert after["a"]["b"] is after["e"]
+
+    # Check if anchors are used
+    out = io.StringIO()
+    ruamel.yaml.YAML().dump(after, out)
+    assert (
+        out.getvalue()
+        == """\
+a: &id001
+  b: &id002
+    c: true
+d: *id001
+e: *id002
+"""
+    )
diff --git a/lib/spack/spack/util/spack_yaml.py b/lib/spack/spack/util/spack_yaml.py
index 9dadc25d82..200025411e 100644
--- a/lib/spack/spack/util/spack_yaml.py
+++ b/lib/spack/spack/util/spack_yaml.py
@@ -20,7 +20,7 @@ import enum
 import functools
 import io
 import re
-from typing import IO, List, Optional
+from typing import IO, Any, Callable, Dict, List, Optional, Union
 
 import ruamel.yaml
 from ruamel.yaml import comments, constructor, emitter, error, representer
@@ -493,6 +493,29 @@ def name_mark(name):
     return error.StringMark(name, None, None, None, None, None)
 
 
+def anchorify(data: Union[dict, list], identifier: Callable[[Any], str] = repr) -> None:
+    """Replace identical dict/list branches in tree with references to earlier instances. The YAML
+    serializer generate anchors for them, resulting in small yaml files."""
+    anchors: Dict[str, Union[dict, list]] = {}
+    queue: List[Union[dict, list]] = [data]
+
+    while queue:
+        item = queue.pop()
+
+        for key, value in item.items() if isinstance(item, dict) else enumerate(item):
+            if not isinstance(value, (dict, list)):
+                continue
+
+            id = identifier(value)
+            anchor = anchors.get(id)
+
+            if anchor is None:
+                anchors[id] = value
+                queue.append(value)
+            else:
+                item[key] = anchor  # replace with reference
+
+
 class SpackYAMLError(spack.error.SpackError):
     """Raised when there are issues with YAML parsing."""