mesh-build-go: a marked package's digest hashes every file of its own directory, Go files its build tags exclude too (issue 1936)

go list leaves a file its build tags exclude (an integration test) in IgnoredGoFiles, which the digest did not hash;
mesh-provisioner-client's unit test globs *_test.go and reads two of them, so a change there reused a stale unit pass.
Review finding F2 on ba49a7f0.

modules/mesh-build-go/cmd/mesh-build-go/digest.go

@@ -44,9 +44,10 @@ const besideVariable = "MESH_CHECK_BESIDE"
 // with ? names a path whose absence is what the tests read (a file that must not be there): it may match nothing.
 //
 // The digest then hashes, besides what the toolchain lists, every line of the marker, every file it names (under its
-// path in the tree, by content) and every file of the package's own directory. A marker that lists nothing, a line
-// that is not a clean relative path, or a line without ? that matches nothing makes the package one that is never kept
-// (its digest a fresh random hex): a list nobody can trust keeps nothing. The exact fix, the files a test opened kept
+// path in the tree, by content) and every file of the package's own directory, Go files included (a file its build tags
+// exclude is one go list does not hash). A marker that lists nothing, a line that is not a clean relative path, or a
+// line without ? that matches nothing makes the package one that is never kept (its digest a fresh random hex): a list
+// nobody can trust keeps nothing. The exact fix, the files a test opened kept
 // from the Go test log with each verdict, replaces the lists (issue 1936).
 const ReadsOutsideMarker = "reads-outside"
 
@@ -557,13 +558,10 @@ func declaredReads(tree string, root *goPackage, cache map[string]string) (lines
 			}
 		}
 	}
-	// The package's own files that are not Go source: its lists, its marker. A catalogue module's are its module's too.
-	err = walkFiles(root.Dir, func(path string) error {
-		if strings.HasSuffix(path, ".go") {
-			return nil
-		}
-		return add(path)
-	})
+	// Every file of the package's own directory: its lists, its marker, and its Go files too, since go list leaves a
+	// file its build tags exclude (an integration test a unit test globs) out of what the digest hashes. A file the
+	// toolchain lists is hashed twice, which changes nothing.
+	err = walkFiles(root.Dir, add)
 	return lines, "", err
 }
 

modules/mesh-build-go/cmd/mesh-build-go/digest_test.go

@@ -241,12 +241,13 @@ func TestAPackageWithTheReadsOutsideMarkerIsKeptOnlyByWhatItLists(t *testing.T)
 		file, text string
 		moves      bool
 	}{
-		{"other.md", "changed\n", false},          // outside the list
-		{"d/d.go", "package d\n\n// D.\n", false}, // a package c's tests do not reach
-		{"docs/one.md", "changed\n", true},        // a listed file
-		{"docs/two.md", "new\n", true},            // a new file the glob matches
-		{"gone", "here\n", true},                  // a path whose absence was read
-		{"c/note.txt", "changed\n", true},         // the package's own file that is not Go
+		{"other.md", "changed\n", false},                                           // outside the list
+		{"d/d.go", "package d\n\n// D.\n", false},                                  // a package c's tests do not reach
+		{"docs/one.md", "changed\n", true},                                         // a listed file
+		{"docs/two.md", "new\n", true},                                             // a new file the glob matches
+		{"gone", "here\n", true},                                                   // a path whose absence was read
+		{"c/note.txt", "changed\n", true},                                          // the package's own file that is not Go
+		{"c/x_integration_test.go", "//go:build integration\n\npackage c\n", true}, // a Go file its build tags leave out (go list's IgnoredGoFiles)
 	} {
 		edit(t, tree, step.file, step.text)
 		after := digests(t, m, "unit")