Don't require a FIFO to be identifiable as such

The jobserver specification [0] currently suggests that the FIFO must be a
genuine FIFO.

For some work we're doing [1][2], we're emulating a FIFO using CUSE/FUSE to allow
tracking when consumers disappear to avoid lost tokens. nixos had a similar
idea in the past too [3].

There doesn't seem to be a good reason to check that any FIFO passed by
the user is actually identifiable as such by `stat()`, so drop the check.

make already does not perform such a check, just the specification isn't
clear about it, so we've asked them to clarify it [4].

[0] https://www.gnu.org/software/make/manual/html_node/POSIX-Jobserver.html
[1] https://codeberg.org/amonakov/guildmaster
[2] https://gitweb.gentoo.org/proj/steve.git/
[3] NixOS/nixpkgs#314888
[4] https://savannah.gnu.org/bugs/index.php?67726

Author: thesamesam

src/jobserver-posix.cc

@@ -26,13 +26,6 @@
 
 namespace {
 
-// Return true if |fd| is a fifo or pipe descriptor.
-bool IsFifoDescriptor(int fd) {
-  struct stat info;
-  int ret = ::fstat(fd, &info);
-  return (ret == 0) && ((info.st_mode & S_IFMT) == S_IFIFO);
-}
-
 // Implementation of Jobserver::Client for Posix systems
 class PosixJobserverClient : public Jobserver::Client {
  public:
@@ -89,11 +82,6 @@ class PosixJobserverClient : public Jobserver::Client {
           std::string("Error opening fifo for reading: ") + strerror(errno);
       return false;
     }
-    if (!IsFifoDescriptor(read_fd_)) {
-      *error = "Not a fifo path: " + fifo_path;
-      // Let destructor close read_fd_.
-      return false;
-    }
     write_fd_ = ::open(fifo_path.c_str(), O_WRONLY | O_NONBLOCK | O_CLOEXEC);
     if (write_fd_ < 0) {
       *error =

src/jobserver_test.cc

@@ -379,21 +379,13 @@ TEST(Jobserver, PosixFifoClientWithWrongPath) {
   ASSERT_GE(fd, 0) << "Could not create file: " << strerror(errno);
   ::close(fd);
 
-  // Create new client instance, passing the file path for the fifo.
+  // Create new client instance, with an empty file path.
   Jobserver::Config config;
   config.mode = Jobserver::Config::kModePosixFifo;
-  config.path = file_path;
-
   std::string error;
   std::unique_ptr<Jobserver::Client> client =
       Jobserver::Client::Create(config, &error);
-  EXPECT_FALSE(client.get());
-  EXPECT_FALSE(error.empty());
-  EXPECT_EQ("Not a fifo path: " + file_path, error);
 
-  // Do the same with an empty file path.
-  error.clear();
-  config.path.clear();
   client = Jobserver::Client::Create(config, &error);
   EXPECT_FALSE(client.get());
   EXPECT_FALSE(error.empty());