Print file summary on checksum validation failure (#35161)

Currently we print "sha256 checksum failed for [file]. Expected X but
got Y".

This PR extends that message with file size and contents info:

"... but got Y. File size = 123456 bytes. Contents = b'abc...def'"

That way we can immediately see if the file was downloaded only
partially, or if we downloaded a text page instead of a binary, etc.

Co-authored-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>

lib/spack/spack/fetch_strategy.py

@@ -95,6 +95,22 @@ def _ensure_one_stage_entry(stage_path):
     return os.path.join(stage_path, stage_entries[0])
 
 
+def _filesummary(path, print_bytes=16):
+    try:
+        n = print_bytes
+        with open(path, "rb") as f:
+            size = os.fstat(f.fileno()).st_size
+            if size <= 2 * n:
+                short_contents = f.read(2 * n)
+            else:
+                short_contents = f.read(n)
+                f.seek(-n, 2)
+                short_contents += b"..." + f.read(n)
+        return size, short_contents
+    except OSError:
+        return 0, b""
+
+
 def fetcher(cls):
     """Decorator used to register fetch strategies."""
     all_strategies.append(cls)
@@ -500,9 +516,14 @@ def check(self):
 
         checker = crypto.Checker(self.digest)
         if not checker.check(self.archive_file):
+            # On failure, provide some information about the file size and
+            # contents, so that we can quickly see what the issue is (redirect
+            # was not followed, empty file, text instead of binary, ...)
+            size, contents = _filesummary(self.archive_file)
             raise ChecksumError(
-                "%s checksum failed for %s" % (checker.hash_name, self.archive_file),
-                "Expected %s but got %s" % (self.digest, checker.sum),
+                f"{checker.hash_name} checksum failed for {self.archive_file}",
+                f"Expected {self.digest} but got {checker.sum}. "
+                f"File size = {size} bytes. Contents = {contents!r}",
             )
 
     @_needs_stage

lib/spack/spack/test/fetch_strategy.py

@@ -5,7 +5,7 @@
 
 import pytest
 
-from spack.fetch_strategy import from_url_scheme
+from spack import fetch_strategy
 
 
 def test_fetchstrategy_bad_url_scheme():
@@ -13,4 +13,14 @@ def test_fetchstrategy_bad_url_scheme():
     unsupported scheme fails as expected."""
 
     with pytest.raises(ValueError):
-        fetcher = from_url_scheme("bogus-scheme://example.com/a/b/c")  # noqa: F841
+        fetcher = fetch_strategy.from_url_scheme("bogus-scheme://example.com/a/b/c")  # noqa: F841
+
+
+def test_filesummary(tmpdir):
+    p = str(tmpdir.join("xyz"))
+    with open(p, "wb") as f:
+        f.write(b"abcdefghijklmnopqrstuvwxyz")
+
+    assert fetch_strategy._filesummary(p, print_bytes=8) == (26, b"abcdefgh...stuvwxyz")
+    assert fetch_strategy._filesummary(p, print_bytes=13) == (26, b"abcdefghijklmnopqrstuvwxyz")
+    assert fetch_strategy._filesummary(p, print_bytes=100) == (26, b"abcdefghijklmnopqrstuvwxyz")