reduce false positive warnings

Author: vanhauser-thc

Changelog.md

@@ -1,4 +1,7 @@
 ### v1.1-dev
+- C/C++ "build is CACHED" warning is confirmed against the artifact's mtime, so a
+  single up-to-date subdirectory in a recursive `make` no longer triggers a false
+  cache warning when the analyzed artifact was actually rebuilt.
 - Indirect resolution precision: escape value-flow follows exact operands
   (PHI/select/GEP/aggregate) and the conservative address-taken fallback fires
   only for genuinely unresolved cast flows, trimming spurious low-confidence

driver/reachability/acquire_c.py

@@ -36,7 +36,7 @@
 import time
 
 from reachability import diagnostics
-from reachability.errors import build_looks_cached, decode, tail
+from reachability.errors import build_is_cached, build_looks_cached, decode, tail
 
 
 class AcquireError(RuntimeError):
@@ -559,10 +559,6 @@ def acquire_c_bitcode(project_dir, tc, artifact=None, build_cmd=None,
         raise AcquireError(f"build failed (exit {rc}){detail}")
 
     cached = _build_looks_cached(build_output)
-    if cached:
-        print("warning: the build is CACHED (nothing was recompiled); the "
-              "extracted bitcode reflects the existing artifact, not this run. "
-              "Rebuild from clean if the target or its flags changed.")
 
     errors = []
 
@@ -622,6 +618,11 @@ def _no_bitcode(base):
             "get-bc could not extract bitcode from any detected artifact:\n  "
             + "\n  ".join(errors))
 
+    if build_is_cached(build_output, art, before):
+        print("warning: the build is CACHED (nothing was recompiled); the "
+              "extracted bitcode reflects the existing artifact, not this run. "
+              "Rebuild from clean if the target or its flags changed.")
+
     if static_libs != "none":
         expanded = _include_static_libs(
             project_dir, art, kind, primary, static_libs,

driver/reachability/errors.py

@@ -1,3 +1,6 @@
+import os
+
+
 def decode(value):
     if value is None:
         return ""
@@ -21,3 +24,21 @@ def build_looks_cached(output):
     )):
         return True
     return "Finished" in text and "Compiling " not in text
+
+
+def build_is_cached(output, artifact_path, build_started_at, tolerance=2.0):
+    """Whether the analyzed artifact predates this build (a genuine cache hit).
+
+    ``build_looks_cached`` only scans build text, so a single up-to-date
+    subdirectory in a recursive ``make`` makes it fire even when other targets
+    recompiled. Confirm the text verdict against the artifact's mtime: if it was
+    written during this build it is fresh, so the marker was a partial-cache
+    false positive. ``tolerance`` absorbs coarse filesystem mtime granularity.
+    Fall back to the text verdict when the artifact cannot be stat'd.
+    """
+    if not build_looks_cached(output):
+        return False
+    try:
+        return os.path.getmtime(artifact_path) < build_started_at - tolerance
+    except OSError:
+        return True

driver/tests/test_acquire_c.py

@@ -90,6 +90,42 @@ def test_build_looks_cached():
     assert not acquire_c._build_looks_cached("cc -c foo.c -o foo.o")
 
 
+def test_build_is_cached_stale_artifact(tmp_path):
+    art = tmp_path / "harness.o"
+    _write(art, b"x")
+    old = os.path.getmtime(art) - 100
+    os.utime(art, (old, old))
+    started = old + 50
+    assert acquire_c.build_is_cached(
+        "make: Nothing to be done for 'all'.", str(art), started)
+
+
+def test_build_is_cached_fresh_artifact_recursive_subdir(tmp_path):
+    # Recursive make: one subdir is up-to-date but the artifact was rebuilt.
+    art = tmp_path / "harness.o"
+    _write(art, b"x")
+    started = os.path.getmtime(art) - 100
+    output = ("  CC       gjobread.o\n"
+              "make[2]: Nothing to be done for 'all'.\n"
+              "  CCLD     harness\n")
+    assert not acquire_c.build_is_cached(output, str(art), started)
+
+
+def test_build_is_cached_requires_marker(tmp_path):
+    art = tmp_path / "harness.o"
+    _write(art, b"x")
+    old = os.path.getmtime(art) - 100
+    os.utime(art, (old, old))
+    assert not acquire_c.build_is_cached("  CC harness.o", str(art), old + 50)
+
+
+def test_build_is_cached_missing_artifact_falls_back(tmp_path):
+    missing = str(tmp_path / "nope.o")
+    assert acquire_c.build_is_cached(
+        "make: Nothing to be done for 'all'.", missing, 0.0)
+    assert not acquire_c.build_is_cached("cc -c foo.c -o foo.o", missing, 0.0)
+
+
 def test_detect_build_cmd_none(tmp_path):
     assert acquire_c.detect_build_cmd(str(tmp_path)) is None