cmd/cover: do not cover statements after panic

Given
	x()
	panic(1)
	y()

the y should not show as covered.

Fixes #10185

Change-Id: Iec61f1b096a888e6727be5f4526508654f5d3c91
Reviewed-on: https://go-review.googlesource.com/8140
Reviewed-by: Robert Griesemer <gri@golang.org>

cmd/cover/cover.go

@@ -550,6 +550,16 @@ func (f *File) endsBasicSourceBlock(s ast.Stmt) bool {
 		return true
 	case *ast.TypeSwitchStmt:
 		return true
+	case *ast.ExprStmt:
+		// Calls to panic change the flow.
+		// We really should verify that "panic" is the predefined function,
+		// but without type checking we can't and the likelihood of it being
+		// an actual problem is vanishingly small.
+		if call, ok := s.X.(*ast.CallExpr); ok {
+			if ident, ok := call.Fun.(*ast.Ident); ok && ident.Name == "panic" && len(call.Args) == 1 {
+				return true
+			}
+		}
 	}
 	found, _ := hasFuncLiteral(s)
 	return found

cmd/cover/testdata/main.go

@@ -58,12 +58,31 @@ func verify() {
 			PASS = false
 		}
 	}
+	verifyPanic()
 	if !PASS {
 		fmt.Fprintf(os.Stderr, "FAIL\n")
 		os.Exit(2)
 	}
 }
 
+// verifyPanic is a special check for the known counter that should be
+// after the panic call in testPanic.
+func verifyPanic() {
+	if coverTest.Count[panicIndex-1] != 1 {
+		// Sanity check for test before panic.
+		fmt.Fprintf(os.Stderr, "bad before panic")
+		PASS = false
+	}
+	if coverTest.Count[panicIndex] != 0 {
+		fmt.Fprintf(os.Stderr, "bad at panic: %d should be 0\n", coverTest.Count[panicIndex])
+		PASS = false
+	}
+	if coverTest.Count[panicIndex+1] != 1 {
+		fmt.Fprintf(os.Stderr, "bad after panic")
+		PASS = false
+	}
+}
+
 // count returns the count and index for the counter at the specified line.
 func count(line uint32) (uint32, int) {
 	// Linear search is fine. Choose perfect fit over approximate.

cmd/cover/testdata/test.go

@@ -22,9 +22,26 @@ func testAll() {
 	testTypeSwitch()
 	testSelect1()
 	testSelect2()
+	testPanic()
 	testEmptySwitches()
 }
 
+// The indexes of the counters in testPanic are known to main.go
+const panicIndex = 3
+
+// This test appears first because the index of its counters is known to main.go
+func testPanic() {
+	defer func() {
+		recover()
+	}()
+	check(LINE, 1)
+	panic("should not get next line")
+	check(LINE, 0) // this is GoCover.Count[panicIndex]
+	// The next counter is in testSimple and it will be non-zero.
+	// If the panic above does not trigger a counter, the test will fail
+	// because GoCover.Count[panicIndex] will be the one in testSimple.
+}
+
 func testSimple() {
 	check(LINE, 1)
 }