fix: handle nil Process when exec command doesn't exist (#16)

Why: otel-cli exec crashes with nil pointer when command not found (issue #8)
Approach: Check for nil Process and ProcessState, return exit 127 to match shell
Learned: Shell returns 127 for "command not found", principle of least surprise
Next: Create PR and close issue

Changes:
- Check child.Process != nil before accessing Pid
- Check child.ProcessState != nil before getting ExitCode
- Return exit code 127 when command fails to start (matches shell behavior)
- Sends span with error status even when command doesn't exist

Tests:
- Added test for exec with non-existent command
- Verifies exit code 127 (same as shell)
- Verifies span is sent with error status
- Verifies no panic/crash

Fixes #8

🤖 Claude <[email protected]>

Author: tobert

data_for_test.go

@@ -1434,4 +1434,38 @@ var suites = []FixtureSuite{
 			},
 		},
 	},
+	// exec with non-existent command should fail gracefully
+	{
+		{
+			Name: "exec non-existent command should not panic",
+			Config: FixtureConfig{
+				CliArgs: []string{"exec", "--endpoint", "{{endpoint}}", "command-that-does-not-exist-anywhere"},
+			},
+			Expect: Results{
+				SpanCount: 1,
+				ExitCode:  127, // exit 127 like shell does for "command not found"
+				Config:    otelcli.DefaultConfig().WithEndpoint("{{endpoint}}"),
+			},
+			CheckFuncs: []CheckFunc{
+				func(t *testing.T, f Fixture, r Results) {
+					// should send a span with error status
+					if r.Span == nil {
+						t.Errorf("expected a span to be sent even when command fails to start")
+						return
+					}
+					if r.Span.Status == nil {
+						t.Errorf("expected span status to be set")
+						return
+					}
+					if r.Span.Status.Code != 2 { // STATUS_CODE_ERROR
+						t.Errorf("expected span status code ERROR (2), got %d", r.Span.Status.Code)
+					}
+					// error message should mention the command failure
+					if !strings.Contains(r.Span.Status.Message, "exec command failed") {
+						t.Errorf("expected error message to contain 'exec command failed', got: %q", r.Span.Status.Message)
+					}
+				},
+			},
+		},
+	},
 }

otelcli/exec.go

@@ -146,8 +146,11 @@ func doExec(cmd *cobra.Command, args []string) {
 
 	// append process attributes
 	span.Attributes = append(span.Attributes, processAttrs...)
-	pidAttrs := processPidAttrs(config, int64(child.Process.Pid), int64(os.Getpid()))
-	span.Attributes = append(span.Attributes, pidAttrs...)
+	// child.Process is nil if the command failed to start (e.g., command not found)
+	if child.Process != nil {
+		pidAttrs := processPidAttrs(config, int64(child.Process.Pid), int64(os.Getpid()))
+		span.Attributes = append(span.Attributes, pidAttrs...)
+	}
 
 	cancelCtxDeadline()
 	close(signals)
@@ -169,7 +172,14 @@ func doExec(cmd *cobra.Command, args []string) {
 	}
 
 	// set the global exit code so main() can grab it and os.Exit() properly
-	Diag.ExecExitCode = child.ProcessState.ExitCode()
+	// ProcessState is nil if the command failed to start
+	if child.ProcessState != nil {
+		Diag.ExecExitCode = child.ProcessState.ExitCode()
+	} else {
+		// command failed to start (e.g., command not found)
+		// use exit code 127 to match shell behavior for "command not found"
+		Diag.ExecExitCode = 127
+	}
 
 	config.PropagateTraceparent(span, os.Stdout)
 }