[clang] [llvm] [LLVM] Add GNU make jobserver support (PR #145131)

[Artem Belevich via llvm-commits]

================
@@ -0,0 +1,257 @@
+//===- llvm/Support/Jobserver.cpp - Jobserver Client Implementation -------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/Jobserver.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Config/llvm-config.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/Error.h"
+#include "llvm/Support/raw_ostream.h"
+
+#include <atomic>
+#include <memory>
+#include <mutex>
+#include <new>
+
+#define DEBUG_TYPE "jobserver"
+
+using namespace llvm;
+
+namespace {
+struct JobserverConfig {
+  enum Mode {
+    None,
+    PosixFifo,
+    PosixPipe,
+    Win32Semaphore,
+  };
+  Mode TheMode = None;
+  std::string Path;
+  int ReadFD = -1;
+  int WriteFD = -1;
+};
+
+/// A helper function that checks if `Input` starts with `Prefix`.
+/// If it does, it removes the prefix from `Input`, assigns the remainder to
+/// `Value`, and returns true. Otherwise, it returns false.
+bool getPrefixedValue(StringRef Input, StringRef Prefix, StringRef &Value) {
+  if (Input.consume_front(Prefix)) {
+    Value = Input;
+    return true;
+  }
+  return false;
+}
+
+/// A helper function to parse a string in the format "R,W" where R and W are
+/// non-negative integers representing file descriptors. It populates the
+/// `ReadFD` and `WriteFD` output parameters. Returns true on success.
+bool getFileDescriptorPair(StringRef Input, int &ReadFD, int &WriteFD) {
+  if (sscanf(Input.str().c_str(), "%d,%d", &ReadFD, &WriteFD) != 2)
+    return false;
+  return ReadFD >= 0 && WriteFD >= 0;
+}
+
+/// Parses the `MAKEFLAGS` environment variable string to find jobserver
+/// arguments. It splits the string into space-separated arguments and searches
+/// for `--jobserver-auth` or `--jobserver-fds`. Based on the value of these
+/// arguments, it determines the jobserver mode (Pipe, FIFO, or Semaphore) and
+/// connection details (file descriptors or path).
+Expected<JobserverConfig> parseNativeMakeFlags(StringRef MakeFlags) {
+  JobserverConfig Config;
+  if (MakeFlags.empty())
+    return Config;
+
+  // Split the MAKEFLAGS string into arguments.
+  SmallVector<StringRef, 8> Args;
+  StringRef S = MakeFlags;
+  while (!S.empty()) {
+    size_t Start = S.find_first_not_of(" \t");
+    if (Start == StringRef::npos)
+      break;
+    S = S.substr(Start);
+    size_t End = S.find_first_of(" \t");
+    if (End == StringRef::npos) {
+      Args.push_back(S);
+      break;
+    }
+    Args.push_back(S.substr(0, End));
+    S = S.substr(End);
+  }
+
+  // If '-n' (dry-run) is present as a legacy flag (not starting with '-'),
+  // disable the jobserver.
+  if (!Args.empty() && !Args[0].starts_with("-") && Args[0].contains('n'))
+    return Config;
+
+  // Iterate through arguments to find jobserver flags.
+  // Note that make may pass multiple --jobserver-auth flags; the last one wins.
+  for (StringRef Arg : Args) {
+    StringRef Value;
+    if (getPrefixedValue(Arg, "--jobserver-auth=", Value)) {
+      int R, W;
+      // Try to parse as a file descriptor pair first.
+      if (getFileDescriptorPair(Value, R, W)) {
+        Config.TheMode = JobserverConfig::PosixPipe;
+        Config.ReadFD = R;
+        Config.WriteFD = W;
----------------
Artem-B wrote:

If we were to combine ReadFD and WriteFD into a struct (or just store them as a pair in the Config), then we could drop R/W temporaries and simplify it a bit:

```
      if (auto Descriptors = getFileDescriptorPair(Value)) {
        Config.TheMode = JobserverConfig::PosixPipe;
        Config.Descriptors = *Descriptors;
```

https://github.com/llvm/llvm-project/pull/145131