bugfix: Issue #170 (partial) set function breakpoints on the first instruction

the entry point of a function is the beginning of the prologue, which can be run multiple times for each invocation of a function if the stack needs to be expanded or the scheduler needs to be run.

proc/proc.go

@@ -172,7 +172,30 @@ func (dbp *Process) FindLocation(str string) (uint64, error) {
 	// Try to lookup by function name
 	fn := dbp.goSymTable.LookupFunc(str)
 	if fn != nil {
-		return fn.Entry, nil
+		// fn.Entry, the entry point of the function, is always a prologue, the prologue may call into the scheduler or grow the stack,
+		// this will result in the breakpoint getting hit multiple times without any apparent program progress inbetween.
+		// In order to avoid this confusing behaviour we try to find the first line of the function and set the breakpoint there.
+		filename, lineno, _ := dbp.goSymTable.PCToLine(fn.Entry)
+		var firstLinePC uint64
+		var err error
+		for {
+			lineno++
+			firstLinePC, _, err = dbp.goSymTable.LineToPC(filename, lineno)
+			if err == nil {
+				break
+			}
+			if _, unk := err.(*gosym.UnknownLineError); !unk {
+				return 0, err
+			}
+		}
+
+		var breakAddr uint64
+		if firstLinePC < fn.End {
+			breakAddr = firstLinePC
+		} else {
+			breakAddr = fn.Entry
+		}
+		return breakAddr, nil
 	}
 
 	// Attempt to parse as number for breakpoint id or raw address

proc/proc_test.go

@@ -278,7 +278,6 @@ func testnext(program string, testcases []nextTest, initialLocation string, t *t
 
 func TestNextGeneral(t *testing.T) {
 	testcases := []nextTest{
-		{17, 19},
 		{19, 20},
 		{20, 23},
 		{23, 24},
@@ -299,7 +298,6 @@ func TestNextGeneral(t *testing.T) {
 
 func TestNextGoroutine(t *testing.T) {
 	testcases := []nextTest{
-		{46, 47},
 		{47, 42},
 	}
 	testnext("testnextprog", testcases, "main.testgoroutine", t)
@@ -307,7 +305,6 @@ func TestNextGoroutine(t *testing.T) {
 
 func TestNextFunctionReturn(t *testing.T) {
 	testcases := []nextTest{
-		{13, 14},
 		{14, 35},
 	}
 	testnext("testnextprog", testcases, "main.helloworld", t)

service/test/integration_test.go

@@ -216,7 +216,6 @@ func testnext(testcases []nextTest, initialLocation string, t *testing.T) {
 
 func TestNextGeneral(t *testing.T) {
 	testcases := []nextTest{
-		{17, 19},
 		{19, 20},
 		{20, 23},
 		{23, 24},
@@ -237,7 +236,6 @@ func TestNextGeneral(t *testing.T) {
 
 func TestNextGoroutine(t *testing.T) {
 	testcases := []nextTest{
-		{46, 47},
 		{47, 42},
 	}
 	testnext(testcases, "main.testgoroutine", t)
@@ -245,7 +243,6 @@ func TestNextGoroutine(t *testing.T) {
 
 func TestNextFunctionReturn(t *testing.T) {
 	testcases := []nextTest{
-		{13, 14},
 		{14, 35},
 	}
 	testnext(testcases, "main.helloworld", t)
@@ -285,7 +282,7 @@ func TestClientServer_breakpointInSeparateGoroutine(t *testing.T) {
 		}
 
 		f, l := state.CurrentThread.File, state.CurrentThread.Line
-		if f != "testthreads.go" && l != 8 {
+		if f != "testthreads.go" && l != 9 {
 			t.Fatal("Program did not hit breakpoint")
 		}
 	})