gopackagesdriver: skip root packages that can't be built (#3806)

**What type of PR is this?**

> Bug fix

**What does this PR do? Why is it needed?**

Omits package IDs from gopackagesdriver's roots list if we can't build
package metadata for them.

**Which issues(s) does this PR fix?**

Fixes #3805

**Other notes for review**

Author: jayconrod

go/tools/gopackagesdriver/bazel.go

@@ -26,6 +26,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"regexp"
 	"strconv"
 	"strings"
 )
@@ -149,18 +150,25 @@ func (b *Bazel) Build(ctx context.Context, args ...string) ([]string, error) {
 	return files, nil
 }
 
+var newlineRe = regexp.MustCompile(`\r?\n`)
+
 func (b *Bazel) Query(ctx context.Context, args ...string) ([]string, error) {
 	output, err := b.run(ctx, "query", args...)
 	if err != nil {
 		return nil, fmt.Errorf("bazel query failed: %w", err)
 	}
 
-	trimmedOutput := strings.TrimSpace(output)
-	if len(trimmedOutput) == 0 {
-		return nil, nil
+	lines := newlineRe.Split(output, -1)
+	r, w := 0, 0
+	for ; r < len(lines); r++ {
+		line := strings.TrimSpace(lines[r])
+		if line != "" {
+			lines[w] = line
+			w++
+		}
 	}
-
-	return strings.Split(trimmedOutput, "\n"), nil
+	lines = lines[:w]
+	return lines, nil
 }
 
 func (b *Bazel) WorkspaceRoot() string {

go/tools/gopackagesdriver/gopackagesdriver_test.go

@@ -27,12 +27,19 @@ go_library(
 )
 
 go_test(
-	name = "hello_test",
-	srcs = [
-		"hello_test.go",
-		"hello_external_test.go",
-	],
-	embed = [":hello"],
+    name = "hello_test",
+    srcs = [
+        "hello_test.go",
+        "hello_external_test.go",
+    ],
+    embed = [":hello"],
+)
+
+go_library(
+    name = "incompatible",
+    srcs = ["incompatible.go"],
+    importpath = "example.com/incompatible",
+    target_compatible_with = ["@platforms//:incompatible"],
 )
 
 -- hello.go --
@@ -58,6 +65,10 @@ import "testing"
 
 func TestHelloExternal(t *testing.T) {}
 
+-- incompatible.go --
+//go:build ignore
+package hello
+
 -- subhello/BUILD.bazel --
 load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
 
@@ -76,7 +87,7 @@ import "os"
 func main() {
 	fmt.Fprintln(os.Stderr, "Subdirectory Hello World!")
 }
-		`,
+`,
 	})
 }
 
@@ -278,8 +289,8 @@ func TestOverlay(t *testing.T) {
 	subhelloPath := path.Join(wd, "subhello/subhello.go")
 
 	expectedImportsPerFile := map[string][]string{
-		helloPath:    []string{"fmt"},
-		subhelloPath: []string{"os", "encoding/json"},
+		helloPath:    {"fmt"},
+		subhelloPath: {"os", "encoding/json"},
 	}
 
 	overlayDriverRequest := DriverRequest{
@@ -324,6 +335,33 @@ func TestOverlay(t *testing.T) {
 	expectSetEquality(t, expectedImportsPerFile[subhelloPath], subhelloPkgImportPaths, "subhello imports")
 }
 
+// TestIncompatible checks that a target that can be queried but not analyzed
+// does not appear in .Roots.
+func TestIncompatible(t *testing.T) {
+	resp := runForTest(t, DriverRequest{}, ".", "./...")
+
+	rootLabels := make(map[string]bool)
+	for _, root := range resp.Roots {
+		rootLabels[root] = true
+	}
+
+	// Verify //:hello is in .Roots and check whether its label starts with
+	// "@@" (bzlmod) or "@" (not bzlmod).
+	var incompatibleLabel string
+	if rootLabels["@@//:hello"] {
+		incompatibleLabel = "@@//:incompatible"
+	} else if rootLabels["@//:hello"] {
+		incompatibleLabel = "@//:incompatible"
+	} else {
+		t.Fatalf("response does not contain //:hello; roots were %s", strings.Join(resp.Roots, ", "))
+	}
+
+	// Verify //:incompatible is NOT in .Roots.
+	if rootLabels[incompatibleLabel] {
+		t.Fatalf("response contains root %s", incompatibleLabel)
+	}
+}
+
 func runForTest(t *testing.T, driverRequest DriverRequest, relativeWorkingDir string, args ...string) driverResponse {
 	t.Helper()
 

go/tools/gopackagesdriver/packageregistry.go

@@ -115,12 +115,18 @@ func (pr *PackageRegistry) Match(labels []string) ([]string, []*FlatPackage) {
 					roots[pkg.ID] = struct{}{}
 				}
 			}
-		} else {
+		} else if _, ok := pr.packagesByID[label]; ok {
 			roots[label] = struct{}{}
 			// If an xtest package exists for this package add it to the roots
 			if _, ok := pr.packagesByID[label+"_xtest"]; ok {
 				roots[label+"_xtest"] = struct{}{}
 			}
+		} else {
+			// Skip a package if we don't have .pkg.json for it.
+			// This happens if 'bazel query' matches the target, but 'bazel build'
+			// can't analyze it, for example, if target_compatible_with is set
+			// with contraints not compatible with the host platform.
+			continue
 		}
 	}