build runner no longer supports -fsingle-threaded

[andrewrk]

After #24588, trying to use zig build on single-threaded systems causes something like this:

[18/19] Linking CXX executable zig2
/boot/system/develop/tools/bin/../lib/gcc/x86_64-unknown-haiku/13.3.0/../../../../x86_64-unknown-haiku/bin/ld: warning: /boot/system/develop/lib/crtn.o: missing .note.GNU-stack section implies executable stack
/boot/system/develop/tools/bin/../lib/gcc/x86_64-unknown-haiku/13.3.0/../../../../x86_64-unknown-haiku/bin/ld: NOTE: This behaviour is deprecated and will be removed in a future version of the linker
[19/19] Building stage3
FAILED: [code=2] stage3/bin/zig /boot/home/proj/zig/build/stage3/bin/zig 
cd /boot/home/proj/zig && /boot/home/proj/zig/build/zig2 build --prefix /boot/home/proj/zig/build/stage3 --zig-lib-dir /boot/home/proj/zig/lib -Dversion-string=0.15.0-dev.1428+e17a050bc -Dtarget=native -Dcpu=native -Denable-llvm -Dconfig_h=/boot/home/proj/zig/build/config.h -Dno-langref -Doptimize=Debug
lib/std/Thread.zig:440:9: error: Cannot spawn thread when building in single-threaded mode
        @compileError("Cannot spawn thread when building in single-threaded mode");
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
referenced by:
    start: lib/std/Build/WebServer.zig:167:39
    main: lib/compiler/build_runner.zig:465:17
    4 reference(s) hidden; use '-freference-trace=6' to see all references
lib/std/Thread.zig:440:9: error: Cannot spawn thread when building in single-threaded mode
        @compileError("Cannot spawn thread when building in single-threaded mode");
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ninja: build stopped: subcommand failed.

The answer is in the reference trace: it's trying to spawn a thread for the web ui without checking whether the host is capable of spawning threads.

[mrjbq7]

By my reading of the code, the run.web_server only is created (and then later .start()) if webui_listen is not null, which is only true for these arguments:

• --webui and --webui=
• --time-report
• --fuzz

and i don't see those in the command line mentioned above?

[andrewrk]

problem lies here:

    main: lib/compiler/build_runner.zig:465:17

[mlugg]

@mrjbq7

and i don't see those in the command line mentioned above?

Whether the arguments are passed doesn't matter, because this is a compile error, not a runtime crash.

[andrewrk]

side note: haiku was defaulted to single-threaded as a workaround in #19572

What approach are we looking for? Disabling the Web UI if the build is single threaded?

[mrjbq7]

I didn't notice it was a compile error -- I was thinking disable that block if builtin.single_threaded, and warn/error if passing command-line argument (e.g., webui_listen != null) and disabled.

Something like?

diff --git a/lib/compiler/build_runner.zig b/lib/compiler/build_runner.zig
index f1a0caf47c..6aa413875c 100644
--- a/lib/compiler/build_runner.zig
+++ b/lib/compiler/build_runner.zig
@@ -462,7 +462,11 @@ pub fn main() !void {
     }) else null;

     if (run.web_server) |*ws| {
-        ws.start() catch |err| fatal("failed to start web server: {s}", .{@errorName(err)});
+        if (!builtin.single_threaded) {
+            ws.start() catch |err| fatal("failed to start web server: {s}", .{@errorName(err)});
+        } else {
+            // XXX: warn or error
+        }
     }

     rebuild: while (true) {

Or perhaps, not .init the web_server at all in that case.

Didn't work until I wrapped both .start() and .finish_build(). We should probably leave the struct as is.

index f1a0caf47c..10df33bae7 100644
--- a/lib/compiler/build_runner.zig
+++ b/lib/compiler/build_runner.zig
@@ -450,19 +450,25 @@ pub fn main() !void {
     try run.thread_pool.init(thread_pool_options);
     defer run.thread_pool.deinit();
 
-    run.web_server = if (webui_listen) |listen_address| .init(.{
-        .gpa = gpa,
-        .thread_pool = &run.thread_pool,
-        .graph = &graph,
-        .all_steps = run.step_stack.keys(),
-        .ttyconf = run.ttyconf,
-        .root_prog_node = main_progress_node,
-        .watch = watch,
-        .listen_address = listen_address,
-    }) else null;
+    if (!builtin.single_threaded) {
+        run.web_server = if (webui_listen) |listen_address| .init(.{
+            .gpa = gpa,
+            .thread_pool = &run.thread_pool,
+            .graph = &graph,
+            .all_steps = run.step_stack.keys(),
+            .ttyconf = run.ttyconf,
+            .root_prog_node = main_progress_node,
+            .watch = watch,
+            .listen_address = listen_address,
+        }) else null;
+    } else {
+        run.web_server = null;
+    }
 
-    if (run.web_server) |*ws| {
-        ws.start() catch |err| fatal("failed to start web server: {s}", .{@errorName(err)});
+    if (!builtin.single_threaded) {
+        if (run.web_server) |*ws| {
+            ws.start() catch |err| fatal("failed to start web server: {s}", .{@errorName(err)});
+        }
     }
 
     rebuild: while (true) {
@@ -481,8 +487,10 @@ pub fn main() !void {
             else => return err,
         };
 
-        if (run.web_server) |*web_server| {
-            web_server.finishBuild(.{ .fuzz = fuzz });
+        if (!builtin.single_threaded) {
+            if (run.web_server) |*web_server| {
+                web_server.finishBuild(.{ .fuzz = fuzz });
+            }
         }
 
         if (!watch and run.web_server == null) {