pkg/proc: support watchpoint on nil interface (#3924)

Documentation/backend_test_health.md

@@ -1,9 +1,9 @@
 Tests skipped by each supported backend:
 
-* 386 skipped = 10
+* 386 skipped = 11
 	* 1 broken
 	* 3 broken - cgo stacktraces
-	* 5 not implemented
+	* 6 not implemented
 	* 1 not working due to optimizations
 * arm64 skipped = 1
 	* 1 broken - global variable symbolication
@@ -14,10 +14,10 @@ Tests skipped by each supported backend:
 	* 1 broken - cgo stacktraces
 * darwin/lldb skipped = 1
 	* 1 upstream issue
-* freebsd skipped = 12
+* freebsd skipped = 13
 	* 2 flaky
 	* 2 follow exec not implemented on freebsd
-	* 6 not implemented
+	* 7 not implemented
 	* 2 not working on freebsd
 * linux/386 skipped = 2
 	* 2 not working on linux/386
@@ -36,24 +36,24 @@ Tests skipped by each supported backend:
 * linux/riscv64 skipped = 2
 	* 1 broken - cgo stacktraces
 	* 1 not working on linux/riscv64
-* loong64 skipped = 8
+* loong64 skipped = 9
 	* 2 broken
 	* 1 broken - global variable symbolication
-	* 5 not implemented
+	* 6 not implemented
 * pie skipped = 2
 	* 2 upstream issue - https://github.com/golang/go/issues/29322
-* ppc64le skipped = 13
+* ppc64le skipped = 14
 	* 6 broken
 	* 1 broken - global variable symbolication
-	* 6 not implemented
-* riscv64 skipped = 7
+	* 7 not implemented
+* riscv64 skipped = 8
 	* 2 broken
 	* 1 broken - global variable symbolication
-	* 4 not implemented
-* windows skipped = 8
+	* 5 not implemented
+* windows skipped = 9
 	* 1 broken
 	* 2 not working on windows
-	* 5 see https://github.com/go-delve/delve/issues/2768
+	* 6 see https://github.com/go-delve/delve/issues/2768
 * windows/arm64 skipped = 5
 	* 3 broken
 	* 1 broken - cgo stacktraces

_fixtures/watchpointInterfaceNil.go

@@ -0,0 +1,15 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+)
+
+func main() {
+	var err error
+	fmt.Println("Error created:", err)
+	err = errors.New("modified error")
+	fmt.Println("Error modified:", err)
+	err = errors.New("final modified error")
+	fmt.Println("Error modified final:", err)
+}

pkg/proc/breakpoints.go

@@ -649,13 +649,10 @@ func (t *Target) SetWatchpoint(logicalID int, scope *EvalScope, expr string, wty
 	if xv.Kind == reflect.Interface {
 		// For interfaces, we want to watch the data they point to
 		// Read the interface to get the data pointer
-		_, data, isnil := xv.readInterface()
+		_, data, _ := xv.readInterface()
 		if xv.Unreadable != nil {
 			return nil, fmt.Errorf("error reading interface %q: %v", expr, xv.Unreadable)
 		}
-		if isnil {
-			return nil, fmt.Errorf("can not watch nil interface %q", expr)
-		}
 		if data == nil {
 			return nil, fmt.Errorf("invalid interface %q", expr)
 		}

pkg/proc/proc_test.go

@@ -5575,6 +5575,46 @@ func TestWatchpointInterface(t *testing.T) {
 	})
 }
 
+func TestWatchpointInterfaceNil(t *testing.T) {
+	skipOn(t, "not implemented", "freebsd")
+	skipOn(t, "not implemented", "386")
+	skipOn(t, "not implemented", "ppc64le")
+	skipOn(t, "not implemented", "riscv64")
+	skipOn(t, "not implemented", "loong64")
+	skipOn(t, "see https://github.com/go-delve/delve/issues/2768", "windows")
+	protest.AllowRecording(t)
+
+	withTestProcess("watchpointInterfaceNil", t, func(p *proc.Target, grp *proc.TargetGroup, fixture protest.Fixture) {
+		// Set breakpoint at line after error is created and printed
+		setFileBreakpoint(p, t, fixture.Source, 10)
+		assertNoError(grp.Continue(), t, "Continue()")
+		assertLineNumber(p, t, 10, "continued to wrong line")
+
+		// Get scope and set watchpoint on interface type
+		scope, err := proc.GoroutineScope(p, p.CurrentThread())
+		assertNoError(err, t, "GoroutineScope")
+
+		// Set watchpoint on the error interface
+		_, err = p.SetWatchpoint(0, scope, "err", proc.WatchWrite, nil)
+		assertNoError(err, t, "SetWatchpoint on interface type")
+
+		// Continue to hit the watchpoint when the error is modified
+		assertNoError(grp.Continue(), t, "Continue to watchpoint")
+
+		// Verify we stopped at the correct line where err is modified
+		assertLineNumberIn(p, t, []int{11, 12}, "stopped at wrong line after interface watchpoint")
+
+		// Continue to hit the watchpoint when the error is modified again
+		assertNoError(grp.Continue(), t, "Continue to watchpoint (2)")
+
+		// Verify we stopped at the correct line where err is modified
+		assertLineNumberIn(p, t, []int{13, 14}, "stopped at wrong line after interface watchpoint (2)")
+
+		// Continue to the end
+		assertNoError(grp.Continue(), t, "Final continue")
+	})
+}
+
 func TestStackwatchClearBug(t *testing.T) {
 	skipOn(t, "not implemented", "freebsd")
 	skipOn(t, "not implemented", "386")