miscompilation regarding pointer to optional slice

[andrewrk]

Zig Version

0.11.0-dev.2148+b3af5d076

Steps to Reproduce and Observed Behavior

Extracted from #14647.

Apply this diff:

diff --git a/lib/std/Build/RunStep.zig b/lib/std/Build/RunStep.zig
index d671ff7ae..30f5ed968 100644
--- a/lib/std/Build/RunStep.zig
+++ b/lib/std/Build/RunStep.zig
@@ -587,8 +587,7 @@ fn runCommand(self: *RunStep, argv: []const []const u8, has_side_effects: bool)
 switch (self.stdio) {
 .check => |checks| for (checks.items) |check| switch (check) {
 .expect_stderr_exact => |expected_bytes| {
- assert(!result.stderr_null);
- if (!mem.eql(u8, expected_bytes, result.stderr)) {
+ if (!mem.eql(u8, expected_bytes, result.stderr.?)) {
 return step.fail(
 \\
 \\========= expected this stderr: =========
@@ -599,14 +598,13 @@ fn runCommand(self: *RunStep, argv: []const []const u8, has_side_effects: bool)
 \\{s}
 , .{
 expected_bytes,
- result.stderr,
+ result.stderr.?,
 try Step.allocPrintCmd(arena, self.cwd, argv),
 });
 }
 },
 .expect_stderr_match => |match| {
- assert(!result.stderr_null);
- if (mem.indexOf(u8, result.stderr, match) == null) {
+ if (mem.indexOf(u8, result.stderr.?, match) == null) {
 return step.fail(
 \\
 \\========= expected to find in stderr: =========
@@ -617,14 +615,13 @@ fn runCommand(self: *RunStep, argv: []const []const u8, has_side_effects: bool)
 \\{s}
 , .{
 match,
- result.stderr,
+ result.stderr.?,
 try Step.allocPrintCmd(arena, self.cwd, argv),
 });
 }
 },
 .expect_stdout_exact => |expected_bytes| {
- assert(!result.stdout_null);
- if (!mem.eql(u8, expected_bytes, result.stdout)) {
+ if (!mem.eql(u8, expected_bytes, result.stdout.?)) {
 return step.fail(
 \\
 \\========= expected this stdout: =========
@@ -635,14 +632,13 @@ fn runCommand(self: *RunStep, argv: []const []const u8, has_side_effects: bool)
 \\{s}
 , .{
 expected_bytes,
- result.stdout,
+ result.stdout.?,
 try Step.allocPrintCmd(arena, self.cwd, argv),
 });
 }
 },
 .expect_stdout_match => |match| {
- assert(!result.stdout_null);
- if (mem.indexOf(u8, result.stdout, match) == null) {
+ if (mem.indexOf(u8, result.stdout.?, match) == null) {
 return step.fail(
 \\
 \\========= expected to find in stdout: =========
@@ -653,7 +649,7 @@ fn runCommand(self: *RunStep, argv: []const []const u8, has_side_effects: bool)
 \\{s}
 , .{
 match,
- result.stdout,
+ result.stdout.?,
 try Step.allocPrintCmd(arena, self.cwd, argv),
 });
 }
@@ -675,12 +671,8 @@ fn runCommand(self: *RunStep, argv: []const []const u8, has_side_effects: bool)
 }
 
 const ChildProcResult = struct {
- // These use boolean flags instead of optionals as a workaround for
- // https://github.com/ziglang/zig/issues/14783
- stdout: []const u8,
- stderr: []const u8,
- stdout_null: bool,
- stderr_null: bool,
+ stdout: ?[]const u8,
+ stderr: ?[]const u8,
 term: std.process.Child.Term,
 elapsed_ns: u64,
 peak_rss: usize,
@@ -725,12 +717,8 @@ fn spawnChildAndCollect(
 });
 var timer = try std.time.Timer.start();
 
- // These are not optionals, as a workaround for
- // https://github.com/ziglang/zig/issues/14783
- var stdout_bytes: []const u8 = undefined;
- var stderr_bytes: []const u8 = undefined;
- var stdout_null = true;
- var stderr_null = true;
+ var stdout_bytes: ?[]const u8 = null;
+ var stderr_bytes: ?[]const u8 = null;
 
 if (child.stdout) |stdout| {
 if (child.stderr) |stderr| {
@@ -749,24 +737,22 @@ fn spawnChildAndCollect(
 
 stdout_bytes = try poller.fifo(.stdout).toOwnedSlice();
 stderr_bytes = try poller.fifo(.stderr).toOwnedSlice();
- stdout_null = false;
- stderr_null = false;
 } else {
 stdout_bytes = try stdout.reader().readAllAlloc(arena, self.max_stdio_size);
- stdout_null = false;
 }
 } else if (child.stderr) |stderr| {
 stderr_bytes = try stderr.reader().readAllAlloc(arena, self.max_stdio_size);
- stderr_null = false;
 }
 
- if (!stderr_null and stderr_bytes.len > 0) {
- const stderr_is_diagnostic = switch (self.stdio) {
- .check => |checks| !checksContainStderr(checks.items),
- else => true,
- };
- if (stderr_is_diagnostic) {
- try self.step.result_error_msgs.append(arena, stderr_bytes);
+ if (stderr_bytes) |stderr| {
+ if (stderr.len > 0) {
+ const stderr_is_diagnostic = switch (self.stdio) {
+ .check => |checks| !checksContainStderr(checks.items),
+ else => true,
+ };
+ if (stderr_is_diagnostic) {
+ try self.step.result_error_msgs.append(arena, stderr);
+ }
 }
 }
 
@@ -776,8 +762,6 @@ fn spawnChildAndCollect(
 return .{
 .stdout = stdout_bytes,
 .stderr = stderr_bytes,
- .stdout_null = stdout_null,
- .stderr_null = stderr_null,
 .term = term,
 .elapsed_ns = elapsed_ns,
 .peak_rss = child.resource_usage_statistics.getMaxRss() orelse 0,

If the diff no longer applies, look for this code:

zig/lib/std/Build/RunStep.zig

Lines 912 to 917 in b4d58e9

	// These use boolean flags instead of optionals as a workaround for
	// https://github.com/ziglang/zig/issues/14783
	stdout: []const u8,
	stderr: []const u8,
	stdout_null: bool,
	stderr_null: bool,

zig/lib/std/Build/RunStep.zig

Lines 1117 to 1122 in b4d58e9

	// These are not optionals, as a workaround for
	// https://github.com/ziglang/zig/issues/14783
	var stdout_bytes: []const u8 = undefined;
	var stderr_bytes: []const u8 = undefined;
	var stdout_null = true;
	var stderr_null = true;

Next, run zig build test-behavior and observe the stderr.? safety check trip, even though non-null was assigned to that variable.

I originally reported this as #14783, which has been fixed. But it seems there is another miscompilation.

Expected Behavior

With the diff applied, zig build test should have all passing tests.