report error if failed process captures stderr (#9293)

When a Spack Executable was configured to capture stderr and the
process failed, the error messages of the process were discarded.
This made it difficult to understand why the process failed. The
exception is now updated to include the stderr of the process when
the Executable captures stderr.

1 files changed, 16 insertions, 6 deletions

diff --git a/lib/spack/spack/util/executable.py b/lib/spack/spack/util/executable.py
index 1817d0a77b..e64845f0ef 100644
--- a/lib/spack/spack/util/executable.py
+++ b/lib/spack/spack/util/executable.py
@@ -186,18 +186,28 @@ class Executable(object):
                 env=env)
             out, err = proc.communicate()
 
-            rc = self.returncode = proc.returncode
-            if fail_on_error and rc != 0 and (rc not in ignore_errors):
-                raise ProcessError('Command exited with status %d:' %
-                                   proc.returncode, cmd_line)
-
+            result = None
             if output is str or error is str:
                 result = ''
                 if output is str:
                     result += to_str(out)
                 if error is str:
                     result += to_str(err)
-                return result
+
+            rc = self.returncode = proc.returncode
+            if fail_on_error and rc != 0 and (rc not in ignore_errors):
+                long_msg = cmd_line
+                if result:
+                    # If the output is not captured in the result, it will have
+                    # been stored either in the specified files (e.g. if
+                    # 'output' specifies a file) or written to the parent's
+                    # stdout/stderr (e.g. if 'output' is not specified)
+                    long_msg += '\n' + result
+
+                raise ProcessError('Command exited with status %d:' %
+                                   proc.returncode, long_msg)
+
+            return result
 
         except OSError as e:
             raise ProcessError(