add an option to compile zig in single-threaded mode

And enable it for Drone CI. I hate to do this, but I need to make
progress on other fronts.

CMakeLists.txt

@@ -75,6 +75,7 @@ set(ZIG_TARGET_TRIPLE "native" CACHE STRING "arch-os-abi to output binaries for"
 set(ZIG_TARGET_MCPU "baseline" CACHE STRING "-mcpu parameter to output binaries for")
 set(ZIG_EXECUTABLE "" CACHE STRING "(when cross compiling) path to already-built zig binary")
 set(ZIG_PREFER_LLVM_CONFIG off CACHE BOOL "(when cross compiling) use llvm-config to find target llvm dependencies if needed")
+set(ZIG_SINGLE_THREADED off CACHE BOOL "limit the zig compiler to use only 1 thread")
 
 find_package(llvm)
 find_package(clang)
@@ -510,10 +511,13 @@ set(ZIG_STAGE2_SOURCES
     "${CMAKE_SOURCE_DIR}/src/Cache.zig"
     "${CMAKE_SOURCE_DIR}/src/Compilation.zig"
     "${CMAKE_SOURCE_DIR}/src/DepTokenizer.zig"
+    "${CMAKE_SOURCE_DIR}/src/Event.zig"
     "${CMAKE_SOURCE_DIR}/src/Module.zig"
     "${CMAKE_SOURCE_DIR}/src/Package.zig"
     "${CMAKE_SOURCE_DIR}/src/RangeSet.zig"
+    "${CMAKE_SOURCE_DIR}/src/ThreadPool.zig"
     "${CMAKE_SOURCE_DIR}/src/TypedValue.zig"
+    "${CMAKE_SOURCE_DIR}/src/WaitGroup.zig"
     "${CMAKE_SOURCE_DIR}/src/astgen.zig"
     "${CMAKE_SOURCE_DIR}/src/clang.zig"
     "${CMAKE_SOURCE_DIR}/src/clang_options.zig"
@@ -713,6 +717,11 @@ if("${CMAKE_BUILD_TYPE}" STREQUAL "Debug")
 else()
     set(ZIG1_RELEASE_ARG -OReleaseFast --strip)
 endif()
+if(ZIG_SINGLE_THREADED)
+  set(ZIG1_SINGLE_THREADED_ARG "--single-threaded")
+else()
+  set(ZIG1_SINGLE_THREADED_ARG "")
+endif()
 
 set(BUILD_ZIG1_ARGS
     "src/stage1.zig"
@@ -722,6 +731,7 @@ set(BUILD_ZIG1_ARGS
     --override-lib-dir "${CMAKE_SOURCE_DIR}/lib"
     "-femit-bin=${ZIG1_OBJECT}"
     "${ZIG1_RELEASE_ARG}"
+    "${ZIG1_SINGLE_THREADED_ARG}"
     -lc
     --pkg-begin build_options "${ZIG_CONFIG_ZIG_OUT}"
     --pkg-end

ci/drone/linux_script

@@ -17,7 +17,8 @@ git config core.abbrev 9
 
 mkdir build
 cd build
-cmake .. -DCMAKE_BUILD_TYPE=Release "-DCMAKE_INSTALL_PREFIX=$DISTDIR" -DZIG_STATIC=ON -DCMAKE_PREFIX_PATH=/deps/local -GNinja
+# TODO figure out why Drone CI is deadlocking and stop passing -DZIG_SINGLE_THREADED=ON
+cmake .. -DCMAKE_BUILD_TYPE=Release "-DCMAKE_INSTALL_PREFIX=$DISTDIR" -DZIG_STATIC=ON -DCMAKE_PREFIX_PATH=/deps/local -GNinja -DZIG_SINGLE_THREADED=ON
 
 samu install
 ./zig build test -Dskip-release -Dskip-non-native

src/Compilation.zig

@@ -1728,7 +1728,7 @@ fn workerUpdateCObject(
         error.AnalysisFail => return,
         else => {
             {
-                var lock = comp.mutex.acquire();
+                const lock = comp.mutex.acquire();
                 defer lock.release();
                 comp.failed_c_objects.ensureCapacity(comp.gpa, comp.failed_c_objects.items().len + 1) catch {
                     fatal("TODO handle this by setting c_object.status = oom failure", .{});
@@ -1759,7 +1759,7 @@ fn updateCObject(comp: *Compilation, c_object: *CObject, c_comp_progress_node: *
 
     if (c_object.clearStatus(comp.gpa)) {
         // There was previous failure.
-        var lock = comp.mutex.acquire();
+        const lock = comp.mutex.acquire();
         defer lock.release();
         comp.failed_c_objects.removeAssertDiscard(c_object);
     }
@@ -1789,10 +1789,12 @@ fn updateCObject(comp: *Compilation, c_object: *CObject, c_comp_progress_node: *
 
     {
         const is_collision = blk: {
-            var lock = comp.mutex.acquire();
+            const bin_digest = man.hash.peekBin();
+
+            const lock = comp.mutex.acquire();
             defer lock.release();
 
-            const gop = try comp.c_object_cache_digest_set.getOrPut(comp.gpa, man.hash.peekBin());
+            const gop = try comp.c_object_cache_digest_set.getOrPut(comp.gpa, bin_digest);
             break :blk gop.found_existing;
         };
         if (is_collision) {
@@ -2211,7 +2213,7 @@ fn failCObj(comp: *Compilation, c_object: *CObject, comptime format: []const u8,
 
 fn failCObjWithOwnedErrorMsg(comp: *Compilation, c_object: *CObject, err_msg: *ErrorMsg) InnerError {
     {
-        var lock = comp.mutex.acquire();
+        const lock = comp.mutex.acquire();
         defer lock.release();
         {
             errdefer err_msg.destroy(comp.gpa);

src/ThreadPool.zig

@@ -25,6 +25,8 @@ pub fn init(self: *ThreadPool, allocator: *std.mem.Allocator) !void {
         .allocator = allocator,
         .threads = &[_]*std.Thread{},
     };
+    if (std.builtin.single_threaded)
+        return;
 
     errdefer self.deinit();
 
@@ -67,6 +69,10 @@ pub fn shutdown(self: *ThreadPool) void {
 }
 
 pub fn spawn(self: *ThreadPool, comptime func: anytype, args: anytype) !void {
+    if (std.builtin.single_threaded) {
+        @call(.{}, func, args);
+        return;
+    }
     const Args = @TypeOf(args);
     const Closure = struct {
         arguments: Args,

src/stage1/zig0.cpp

@@ -266,6 +266,7 @@ int main(int argc, char **argv) {
     TargetSubsystem subsystem = TargetSubsystemAuto;
     const char *override_lib_dir = nullptr;
     const char *mcpu = nullptr;
+    bool single_threaded = false;
 
     for (int i = 1; i < argc; i += 1) {
         char *arg = argv[i];
@@ -281,6 +282,8 @@ int main(int argc, char **argv) {
                 optimize_mode = BuildModeSafeRelease;
             } else if (strcmp(arg, "-OReleaseSmall") == 0) {
                 optimize_mode = BuildModeSmallRelease;
+            } else if (strcmp(arg, "--single-threaded") == 0) {
+                single_threaded = true;
             } else if (strcmp(arg, "--help") == 0) {
                 return print_full_usage(arg0, stdout, EXIT_SUCCESS);
             } else if (strcmp(arg, "--strip") == 0) {
@@ -469,6 +472,7 @@ int main(int argc, char **argv) {
     stage1->link_libcpp = link_libcpp;
     stage1->subsystem = subsystem;
     stage1->pic = true;
+    stage1->is_single_threaded = single_threaded;
 
     zig_stage1_build_object(stage1);