feat(librariangen): add execv package (#3932)

This will be used for executing commands like protoc.

Copied from
https://github.com/googleapis/google-cloud-go/tree/main/internal/librariangen/execv.

Author: meltsufin

internal/librariangen/execv/execv.go

@@ -0,0 +1,51 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package execv
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log/slog"
+	"os"
+	"os/exec"
+	"strings"
+)
+
+// Run executes a command in a specified working directory and logs its output.
+func Run(ctx context.Context, args []string, workingDir string) error {
+	cmd := exec.CommandContext(ctx, args[0], args[1:]...)
+	cmd.Env = os.Environ()
+	cmd.Dir = workingDir
+	slog.Debug("librariangen: running command", "command", strings.Join(cmd.Args, " "), "dir", cmd.Dir)
+
+	output, err := cmd.Output()
+	if len(output) > 0 {
+		slog.Debug("librariangen: command stdout", "output", string(output))
+	}
+	if err != nil {
+		var exitErr *exec.ExitError
+		if errors.As(err, &exitErr) {
+			// The command ran and exited with a non-zero exit code.
+			if len(exitErr.Stderr) > 0 {
+				slog.Debug("librariangen: command stderr", "output", string(exitErr.Stderr))
+			}
+			return fmt.Errorf("librariangen: command failed with exit error: %s: %w", exitErr.Stderr, err)
+		}
+		// Another error occurred (e.g., command not found).
+		return fmt.Errorf("librariangen: command failed: %w", err)
+	}
+	return nil
+}

internal/librariangen/execv/execv_test.go

@@ -0,0 +1,79 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package execv
+
+import (
+	"context"
+	"errors"
+	"os/exec"
+	"strings"
+	"testing"
+)
+
+func TestRun(t *testing.T) {
+	tests := []struct {
+		name         string
+		args         []string
+		wantErr      bool
+		wantExit     int
+		wantInStderr string
+	}{
+		{
+			name:    "valid command",
+			args:    []string{"echo", "hello"},
+			wantErr: false,
+		},
+		{
+			name:    "invalid command",
+			args:    []string{"command-that-does-not-exist"},
+			wantErr: true,
+		},
+		{
+			name:     "command with non-zero exit",
+			args:     []string{"sh", "-c", "exit 1"},
+			wantErr:  true,
+			wantExit: 1,
+		},
+		{
+			name:         "command with stderr output",
+			args:         []string{"sh", "-c", "echo 'test error' >&2; exit 1"},
+			wantErr:      true,
+			wantExit:     1,
+			wantInStderr: "test error",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := Run(context.Background(), tt.args, ".")
+			if (err != nil) != tt.wantErr {
+				t.Fatalf("Run() error = %v, wantErr %v", err, tt.wantErr)
+			}
+
+			if !tt.wantErr {
+				return
+			}
+
+			var exitErr *exec.ExitError
+			if errors.As(err, &exitErr) {
+				if tt.wantExit != 0 && exitErr.ExitCode() != tt.wantExit {
+					t.Errorf("Run() exit code = %d, want %d", exitErr.ExitCode(), tt.wantExit)
+				}
+				if tt.wantInStderr != "" && !strings.Contains(string(exitErr.Stderr), tt.wantInStderr) {
+					t.Errorf("Run() stderr = %q, want contains %q", string(exitErr.Stderr), tt.wantInStderr)
+				}
+			}
+		})
+	}
+}