Fix Go extractor to extract root internal test files

When CODEQL_EXTRACTOR_GO_OPTION_EXTRACT_TESTS=true is set, the Go
extractor was incorrectly skipping internal test files (package foo)
at repository roots when the project contains nested test packages.

Root Cause:
The extractor selected package variants by longest ID string, but this
heuristic fails when nested packages have tests. For a package like
"github.com/go-git/go-git/v6", packages.Load returns multiple variants:

1. "github.com/go-git/go-git/v6" (19 files, production only)
2. "github.com/go-git/go-git/v6 [github.com/go-git/go-git/v6.test]"
   (39 files, production + 20 root tests) ← Should select this
3. "github.com/go-git/go-git/v6 [github.com/go-git/go-git/v6/plumbing/format/packfile.test]"
   (19 files, test dependency) ← Was incorrectly selected (longest string)

The old logic selected variant #3 (76 chars) over #2 (68 chars),
causing 20 root test files to be missing from the database.

Fix:
Replace string length comparison with a better heuristic that prefers:
1. Exact test packages (e.g., "pkg [pkg.test]") over nested dependencies
2. Packages with more Syntax nodes (more files to extract)
3. String length as a tiebreaker

This ensures the extractor selects the variant with the most complete
test coverage, particularly for root-level internal tests.

Testing:
- Added comprehensive unit tests covering the selection logic
- Tests simulate the real-world go-git scenario
- All tests pass

Impact:
Root-level external tests (package foo_test) were already extracted
correctly. This fix ensures internal tests (package foo) at the root
are now also extracted when they exist alongside nested test packages.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

go/extractor/extractor.go

@@ -59,6 +59,44 @@ func init() {
 	}
 }
 
+// isExactTestPackage checks if a package ID represents an exact test match.
+// Returns true for IDs like "github.com/foo/bar [github.com/foo/bar.test]"
+// Returns false for IDs like "github.com/foo/bar [github.com/foo/bar/nested.test]"
+func isExactTestPackage(pkg *packages.Package) bool {
+	// Test packages have IDs in the format: "pkgpath [pkgpath.test]"
+	// or for nested test dependencies: "pkgpath [pkgpath/nested.test]"
+	if !strings.Contains(pkg.ID, " [") {
+		return false
+	}
+	expectedTestID := pkg.PkgPath + " [" + pkg.PkgPath + ".test]"
+	return pkg.ID == expectedTestID
+}
+
+// isBetterPackage determines if pkg is a better choice than current for extraction.
+// Preferences:
+// 1. Exact test package (e.g., "pkg [pkg.test]") over nested test dependencies
+// 2. More Syntax nodes (more files to extract)
+// 3. Longer ID string as tiebreaker
+func isBetterPackage(pkg, current *packages.Package) bool {
+	pkgIsExact := isExactTestPackage(pkg)
+	currentIsExact := isExactTestPackage(current)
+
+	// Prefer exact test packages
+	if pkgIsExact != currentIsExact {
+		return pkgIsExact
+	}
+
+	// Prefer packages with more syntax nodes (more files)
+	pkgSyntaxCount := len(pkg.Syntax)
+	currentSyntaxCount := len(current.Syntax)
+	if pkgSyntaxCount != currentSyntaxCount {
+		return pkgSyntaxCount > currentSyntaxCount
+	}
+
+	// Fall back to string length
+	return len(pkg.ID) > len(current.ID)
+}
+
 // ExtractWithFlags extracts the packages specified by the given patterns and build flags
 func ExtractWithFlags(buildFlags []string, patterns []string, extractTests bool, sourceRoot string) error {
 	startTime := time.Now()
@@ -153,20 +191,22 @@ func ExtractWithFlags(buildFlags []string, patterns []string, extractTests bool,
 
 	pkgsNotFound := make([]string, 0, len(pkgs))
 
-	// Build a map from package paths to their longest IDs--
+	// Build a map from package paths to their best IDs--
 	// in the context of a `go test -c` compilation, we will see the same package more than
 	// once, with IDs like "abc.com/pkgname [abc.com/pkgname.test]" to distinguish the version
 	// that contains and is used by test code.
-	// For our purposes it is simplest to just ignore the non-test version, since the test
-	// version seems to be a superset of it.
-	longestPackageIds := make(map[string]string)
+	// We prefer the version with the most complete test coverage, which is typically:
+	// 1. The exact test package (e.g., "pkg [pkg.test]") over nested test dependencies
+	// 2. The package with the most Syntax nodes (most files to extract)
+	// 3. The longest ID string as a tiebreaker
+	bestPackageIds := make(map[string]*packages.Package)
 	packages.Visit(pkgs, nil, func(pkg *packages.Package) {
-		if longestIDSoFar, present := longestPackageIds[pkg.PkgPath]; present {
-			if len(pkg.ID) > len(longestIDSoFar) {
-				longestPackageIds[pkg.PkgPath] = pkg.ID
+		if bestSoFar, present := bestPackageIds[pkg.PkgPath]; present {
+			if isBetterPackage(pkg, bestSoFar) {
+				bestPackageIds[pkg.PkgPath] = pkg
 			}
 		} else {
-			longestPackageIds[pkg.PkgPath] = pkg.ID
+			bestPackageIds[pkg.PkgPath] = pkg
 		}
 	})
 
@@ -257,15 +297,15 @@ func ExtractWithFlags(buildFlags []string, patterns []string, extractTests bool,
 	// extract AST information for all packages
 	packages.Visit(pkgs, nil, func(pkg *packages.Package) {
 
-		// If this is a variant of a package that also occurs with a longer ID, skip it;
+		// If this is a variant of a package that also occurs with a better ID, skip it;
 		// otherwise we would extract the same file more than once including extracting the
 		// body of methods twice, causing database inconsistencies.
 		//
-		// We prefer the version with the longest ID because that is (so far as I know) always
-		// the version that defines more entities -- the only case I'm aware of being a test
-		// variant of a package, which includes test-only functions in addition to the complete
-		// contents of the main variant.
-		if pkg.ID != longestPackageIds[pkg.PkgPath] {
+		// We prefer the version with the most complete test coverage, prioritizing:
+		// 1. Exact test packages (e.g., "pkg [pkg.test]") over nested test dependencies
+		// 2. Packages with more Syntax nodes (more files to extract)
+		// 3. Longer ID strings as a tiebreaker
+		if pkg.ID != bestPackageIds[pkg.PkgPath].ID {
 			return
 		}
 

go/extractor/extractor_test.go

@@ -0,0 +1,255 @@
+package extractor
+
+import (
+	"go/ast"
+	"testing"
+
+	"golang.org/x/tools/go/packages"
+)
+
+func TestIsExactTestPackage(t *testing.T) {
+	tests := []struct {
+		name     string
+		pkgID    string
+		pkgPath  string
+		expected bool
+	}{
+		{
+			name:     "exact test package",
+			pkgID:    "github.com/foo/bar [github.com/foo/bar.test]",
+			pkgPath:  "github.com/foo/bar",
+			expected: true,
+		},
+		{
+			name:     "nested test package",
+			pkgID:    "github.com/foo/bar [github.com/foo/bar/nested.test]",
+			pkgPath:  "github.com/foo/bar",
+			expected: false,
+		},
+		{
+			name:     "deeply nested test package",
+			pkgID:    "github.com/go-git/go-git/v6 [github.com/go-git/go-git/v6/plumbing/format/packfile.test]",
+			pkgPath:  "github.com/go-git/go-git/v6",
+			expected: false,
+		},
+		{
+			name:     "exact test package with version",
+			pkgID:    "github.com/go-git/go-git/v6 [github.com/go-git/go-git/v6.test]",
+			pkgPath:  "github.com/go-git/go-git/v6",
+			expected: true,
+		},
+		{
+			name:     "non-test package",
+			pkgID:    "github.com/foo/bar",
+			pkgPath:  "github.com/foo/bar",
+			expected: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			pkg := &packages.Package{
+				ID:      tt.pkgID,
+				PkgPath: tt.pkgPath,
+			}
+			result := isExactTestPackage(pkg)
+			if result != tt.expected {
+				t.Errorf("isExactTestPackage(%q) = %v, want %v", tt.pkgID, result, tt.expected)
+			}
+		})
+	}
+}
+
+func TestIsBetterPackage(t *testing.T) {
+	// Helper to create a package with specified properties
+	makePkg := func(id, path string, syntaxCount int) *packages.Package {
+		syntax := make([]*ast.File, syntaxCount)
+		return &packages.Package{
+			ID:      id,
+			PkgPath: path,
+			Syntax:  syntax,
+		}
+	}
+
+	tests := []struct {
+		name     string
+		pkg      *packages.Package
+		current  *packages.Package
+		expected bool // true if pkg is better than current
+	}{
+		{
+			name: "exact test package beats nested test package",
+			pkg: makePkg(
+				"github.com/go-git/go-git/v6 [github.com/go-git/go-git/v6.test]",
+				"github.com/go-git/go-git/v6",
+				39, // 19 production + 20 test files
+			),
+			current: makePkg(
+				"github.com/go-git/go-git/v6 [github.com/go-git/go-git/v6/plumbing/format/packfile.test]",
+				"github.com/go-git/go-git/v6",
+				19, // production files only
+			),
+			expected: true,
+		},
+		{
+			name: "nested test package loses to exact test package",
+			pkg: makePkg(
+				"github.com/go-git/go-git/v6 [github.com/go-git/go-git/v6/plumbing/format/packfile.test]",
+				"github.com/go-git/go-git/v6",
+				19,
+			),
+			current: makePkg(
+				"github.com/go-git/go-git/v6 [github.com/go-git/go-git/v6.test]",
+				"github.com/go-git/go-git/v6",
+				39,
+			),
+			expected: false,
+		},
+		{
+			name: "more syntax nodes wins when both are exact tests",
+			pkg: makePkg(
+				"github.com/foo/bar [github.com/foo/bar.test]",
+				"github.com/foo/bar",
+				50,
+			),
+			current: makePkg(
+				"github.com/foo/bar [github.com/foo/bar.test]",
+				"github.com/foo/bar",
+				30,
+			),
+			expected: true,
+		},
+		{
+			name: "fewer syntax nodes loses when both are exact tests",
+			pkg: makePkg(
+				"github.com/foo/bar [github.com/foo/bar.test]",
+				"github.com/foo/bar",
+				30,
+			),
+			current: makePkg(
+				"github.com/foo/bar [github.com/foo/bar.test]",
+				"github.com/foo/bar",
+				50,
+			),
+			expected: false,
+		},
+		{
+			name: "more syntax nodes wins when both are nested tests",
+			pkg: makePkg(
+				"github.com/foo/bar [github.com/foo/bar/pkg1.test]",
+				"github.com/foo/bar",
+				25,
+			),
+			current: makePkg(
+				"github.com/foo/bar [github.com/foo/bar/pkg2.test]",
+				"github.com/foo/bar",
+				20,
+			),
+			expected: true,
+		},
+		{
+			name: "longer ID wins when same syntax count",
+			pkg: makePkg(
+				"github.com/foo/bar [github.com/foo/bar/verylongpackagename.test]",
+				"github.com/foo/bar",
+				20,
+			),
+			current: makePkg(
+				"github.com/foo/bar [github.com/foo/bar/short.test]",
+				"github.com/foo/bar",
+				20,
+			),
+			expected: true,
+		},
+		{
+			name: "test package beats non-test with same syntax count",
+			pkg: makePkg(
+				"github.com/foo/bar [github.com/foo/bar.test]",
+				"github.com/foo/bar",
+				20,
+			),
+			current: makePkg(
+				"github.com/foo/bar",
+				"github.com/foo/bar",
+				20,
+			),
+			expected: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := isBetterPackage(tt.pkg, tt.current)
+			if result != tt.expected {
+				t.Errorf("isBetterPackage() = %v, want %v\n  pkg:     %q (%d syntax nodes)\n  current: %q (%d syntax nodes)",
+					result, tt.expected,
+					tt.pkg.ID, len(tt.pkg.Syntax),
+					tt.current.ID, len(tt.current.Syntax))
+			}
+		})
+	}
+}
+
+// TestPackageSelectionRealWorld simulates the real-world go-git scenario
+func TestPackageSelectionRealWorld(t *testing.T) {
+	// Simulate the actual packages.Load result for go-git repository
+	// when EXTRACT_TESTS=true
+	pkgs := []*packages.Package{
+		// Production package only
+		{
+			ID:      "github.com/go-git/go-git/v6",
+			PkgPath: "github.com/go-git/go-git/v6",
+			Syntax:  make([]*ast.File, 19), // 19 production files
+		},
+		// Root test package - this is what we want!
+		{
+			ID:      "github.com/go-git/go-git/v6 [github.com/go-git/go-git/v6.test]",
+			PkgPath: "github.com/go-git/go-git/v6",
+			Syntax:  make([]*ast.File, 39), // 19 production + 20 test files
+		},
+		// Nested test dependency 1
+		{
+			ID:      "github.com/go-git/go-git/v6 [github.com/go-git/go-git/v6/plumbing/format/packfile.test]",
+			PkgPath: "github.com/go-git/go-git/v6",
+			Syntax:  make([]*ast.File, 19), // production files only (dependency)
+		},
+		// Nested test dependency 2
+		{
+			ID:      "github.com/go-git/go-git/v6 [github.com/go-git/go-git/v6/plumbing/object.test]",
+			PkgPath: "github.com/go-git/go-git/v6",
+			Syntax:  make([]*ast.File, 19), // production files only (dependency)
+		},
+	}
+
+	// Simulate the bestPackageIds selection logic
+	bestPackageIds := make(map[string]*packages.Package)
+	for _, pkg := range pkgs {
+		if bestSoFar, present := bestPackageIds[pkg.PkgPath]; present {
+			if isBetterPackage(pkg, bestSoFar) {
+				bestPackageIds[pkg.PkgPath] = pkg
+			}
+		} else {
+			bestPackageIds[pkg.PkgPath] = pkg
+		}
+	}
+
+	// Verify the correct package was selected
+	selected := bestPackageIds["github.com/go-git/go-git/v6"]
+	expectedID := "github.com/go-git/go-git/v6 [github.com/go-git/go-git/v6.test]"
+	expectedSyntaxCount := 39
+
+	if selected.ID != expectedID {
+		t.Errorf("Wrong package selected!\n  got:  %q (%d syntax nodes)\n  want: %q (%d syntax nodes)",
+			selected.ID, len(selected.Syntax),
+			expectedID, expectedSyntaxCount)
+	}
+
+	if len(selected.Syntax) != expectedSyntaxCount {
+		t.Errorf("Wrong syntax count: got %d, want %d", len(selected.Syntax), expectedSyntaxCount)
+	}
+
+	// Verify it's recognized as an exact test package
+	if !isExactTestPackage(selected) {
+		t.Errorf("Selected package %q should be recognized as exact test package", selected.ID)
+	}
+}