Fixes to runTerraformCmd for race conditions

 - Use waitgroups for more readability
 - Improve handling errors from writeOutput
 - Finish reading from pipes before calling cmd.Wait - fixes a race condition
 that leads to an error :`read |0: file already closed`
 - Because now waiting for pipes to finish reading, need to update waitGroup to
 close buf.Read on context cancel. Otherwise buf.Read blocks until next line
 before stopping. Causes TestContext_sleepTimeoutExpired takes a little too long
 to cancel (~20s)

tfexec/cmd.go

@@ -233,7 +233,21 @@ func mergeWriters(writers ...io.Writer) io.Writer {
 	return io.MultiWriter(compact...)
 }
 
-func writeOutput(r io.ReadCloser, w io.Writer) error {
+func writeOutput(ctx context.Context, r io.ReadCloser, w io.Writer) error {
+	// ReadBytes will block until bytes are read, which can cause a delay in
+	// returning even if the command's context has been canceled. Use a separate
+	// goroutine to prompt ReadBytes to return on cancel
+	closeCtx, closeCancel := context.WithCancel(ctx)
+	defer closeCancel()
+	go func() {
+		select {
+		case <-ctx.Done():
+			r.Close()
+		case <-closeCtx.Done():
+			return
+		}
+	}()
+
 	buf := bufio.NewReader(r)
 	for {
 		line, err := buf.ReadBytes('\n')

tfexec/cmd_default.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"os/exec"
 	"strings"
+	"sync"
 )
 
 func (tf *Terraform) runTerraformCmd(ctx context.Context, cmd *exec.Cmd) error {
@@ -46,15 +47,24 @@ func (tf *Terraform) runTerraformCmd(ctx context.Context, cmd *exec.Cmd) error {
 		return tf.wrapExitError(ctx, err, "")
 	}
 
-	exitChLen := 2
-	exitCh := make(chan error, exitChLen)
+	var errStdout, errStderr error
+	var wg sync.WaitGroup
+	wg.Add(1)
 	go func() {
-		exitCh <- writeOutput(stdoutPipe, stdoutWriter)
+		defer wg.Done()
+		errStdout = writeOutput(ctx, stdoutPipe, stdoutWriter)
 	}()
+
+	wg.Add(1)
 	go func() {
-		exitCh <- writeOutput(stderrPipe, stderrWriter)
+		defer wg.Done()
+		errStderr = writeOutput(ctx, stderrPipe, stderrWriter)
 	}()
 
+	// Reads from pipes must be completed before calling cmd.Wait(). Otherwise
+	// can cause a race condition
+	wg.Wait()
+
 	err = cmd.Wait()
 	if err == nil && ctx.Err() != nil {
 		err = ctx.Err()
@@ -63,16 +73,13 @@ func (tf *Terraform) runTerraformCmd(ctx context.Context, cmd *exec.Cmd) error {
 		return tf.wrapExitError(ctx, err, errBuf.String())
 	}
 
-	// Wait for the logs to finish writing
-	counter := 0
-	for {
-		counter++
-		err := <-exitCh
-		if err != nil && err != context.Canceled {
-			return tf.wrapExitError(ctx, err, errBuf.String())
-		}
-		if counter >= exitChLen {
-			return ctx.Err()
-		}
+	// Return error if there was an issue reading the std out/err
+	if errStdout != nil && ctx.Err() != nil {
+		return tf.wrapExitError(ctx, errStdout, errBuf.String())
 	}
+	if errStderr != nil && ctx.Err() != nil {
+		return tf.wrapExitError(ctx, errStderr, errBuf.String())
+	}
+
+	return nil
 }

tfexec/cmd_linux.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"os/exec"
 	"strings"
+	"sync"
 	"syscall"
 )
 
@@ -51,15 +52,24 @@ func (tf *Terraform) runTerraformCmd(ctx context.Context, cmd *exec.Cmd) error {
 		return tf.wrapExitError(ctx, err, "")
 	}
 
-	exitChLen := 2
-	exitCh := make(chan error, exitChLen)
+	var errStdout, errStderr error
+	var wg sync.WaitGroup
+	wg.Add(1)
 	go func() {
-		exitCh <- writeOutput(stdoutPipe, stdoutWriter)
+		defer wg.Done()
+		errStdout = writeOutput(ctx, stdoutPipe, stdoutWriter)
 	}()
+
+	wg.Add(1)
 	go func() {
-		exitCh <- writeOutput(stderrPipe, stderrWriter)
+		defer wg.Done()
+		errStderr = writeOutput(ctx, stderrPipe, stderrWriter)
 	}()
 
+	// Reads from pipes must be completed before calling cmd.Wait(). Otherwise
+	// can cause a race condition
+	wg.Wait()
+
 	err = cmd.Wait()
 	if err == nil && ctx.Err() != nil {
 		err = ctx.Err()
@@ -68,16 +78,13 @@ func (tf *Terraform) runTerraformCmd(ctx context.Context, cmd *exec.Cmd) error {
 		return tf.wrapExitError(ctx, err, errBuf.String())
 	}
 
-	// Wait for the logs to finish writing
-	counter := 0
-	for {
-		counter++
-		err := <-exitCh
-		if err != nil && err != context.Canceled {
-			return tf.wrapExitError(ctx, err, errBuf.String())
-		}
-		if counter >= exitChLen {
-			return ctx.Err()
-		}
+	// Return error if there was an issue reading the std out/err
+	if errStdout != nil && ctx.Err() != nil {
+		return tf.wrapExitError(ctx, errStdout, errBuf.String())
 	}
+	if errStderr != nil && ctx.Err() != nil {
+		return tf.wrapExitError(ctx, errStderr, errBuf.String())
+	}
+
+	return nil
 }