libutil/file-descriptor: handle EAGAIN in read/write operations

We now see exception beeing thrown when remote building in master
because of writing to a non-blocking file descriptor from our json logger.

> #0  0x00007f2ea97aea9c in __pthread_kill_implementation () from /nix/store/wn7v2vhyyyi6clcyn0s9ixvl7d4d87ic-glibc-2.40-36/lib/libc.so.6
> #1  0x00007f2ea975c576 in raise () from /nix/store/wn7v2vhyyyi6clcyn0s9ixvl7d4d87ic-glibc-2.40-36/lib/libc.so.6
> #2  0x00007f2ea9744935 in abort () from /nix/store/wn7v2vhyyyi6clcyn0s9ixvl7d4d87ic-glibc-2.40-36/lib/libc.so.6
> #3  0x00007f2ea99e8c2b in __gnu_cxx::__verbose_terminate_handler() [clone .cold] () from /nix/store/ybjcla5bhj8g1y84998pn4a2drfxybkv-gcc-13.3.0-lib/lib/libstdc++.so.6
> #4  0x00007f2ea99f820a in __cxxabiv1::__terminate(void (*)()) () from /nix/store/ybjcla5bhj8g1y84998pn4a2drfxybkv-gcc-13.3.0-lib/lib/libstdc++.so.6
> #5  0x00007f2ea99f8275 in std::terminate() () from /nix/store/ybjcla5bhj8g1y84998pn4a2drfxybkv-gcc-13.3.0-lib/lib/libstdc++.so.6
> #6  0x00007f2ea99f84c7 in __cxa_throw () from /nix/store/ybjcla5bhj8g1y84998pn4a2drfxybkv-gcc-13.3.0-lib/lib/libstdc++.so.6
> #7  0x00007f2eaa5035c2 in nix::writeFull (fd=2, s=..., allowInterrupts=true) at ../unix/file-descriptor.cc:43
> #8  0x00007f2eaa5633c4 in nix::JSONLogger::write (this=this@entry=0x249a7d40, json=...) at /nix/store/4krab2h0hd4wvxxmscxrw21pl77j4i7j-gcc-13.3.0/include/c++/13.3.0/bits/char_traits.h:358
> #9  0x00007f2eaa5658d7 in nix::JSONLogger::logEI (this=<optimized out>, ei=...) at ../logging.cc:242
> #10 0x00007f2ea9c5d048 in nix::Logger::logEI (ei=..., lvl=nix::lvlError, this=0x249a7d40) at /nix/store/a7cq5bqh0ryvnkv4m19ffchnvi8l9qx6-nix-util-2.27.0-dev/include/nix/logging.hh:108
> #11 nix::handleExceptions (programName="nix", fun=...) at ../shared.cc:343
> #12 0x0000000000465b1f in main (argc=<optimized out>, argv=<optimized out>) at /nix/store/4krab2h0hd4wvxxmscxrw21pl77j4i7j-gcc-13.3.0/include/c++/13.3.0/bits/allocator.h:163
> (gdb) frame 10
> #10 0x00007f2ea9c5d048 in nix::Logger::logEI (ei=..., lvl=nix::lvlError, this=0x249a7d40) at /nix/store/a7cq5bqh0ryvnkv4m19ffchnvi8l9qx6-nix-util-2.27.0-dev/include/nix/logging.hh:108
> 108             logEI(ei);

So far only drainFD sets the non-blocking flag on a "readable" file descriptor,
while this is a "writeable" file descriptor.
It's not clear to me yet, why we see logs after that point, but it's
also not that bad to handle EAGAIN in read/write functions after all.

Author: Mic92

src/libutil/unix/file-descriptor.cc

@@ -5,9 +5,27 @@
 
 #include <fcntl.h>
 #include <unistd.h>
+#include <poll.h>
 
 namespace nix {
 
+namespace {
+
+// This function is needed to handle non-blocking reads/writes. This is needed in the buildhook, because
+// somehow the json logger file descriptor ends up beeing non-blocking and breaks remote-building.
+// TODO: get rid of buildhook and remove this function again (https://github.com/NixOS/nix/issues/12688)
+void pollFD(int fd, int events)
+{
+    struct pollfd pfd;
+    pfd.fd = fd;
+    pfd.events = events;
+    int ret = poll(&pfd, 1, -1);
+    if (ret == -1) {
+        throw SysError("poll on file descriptor failed");
+    }
+}
+}
+
 std::string readFile(int fd)
 {
     struct stat st;
@@ -17,14 +35,18 @@ std::string readFile(int fd)
     return drainFD(fd, true, st.st_size);
 }
 
-
 void readFull(int fd, char * buf, size_t count)
 {
     while (count) {
         checkInterrupt();
         ssize_t res = read(fd, buf, count);
         if (res == -1) {
-            if (errno == EINTR) continue;
+            switch (errno) {
+            case EINTR: continue;
+            case EAGAIN:
+                pollFD(fd, POLLIN);
+                continue;
+            }
             throw SysError("reading from file");
         }
         if (res == 0) throw EndOfFile("unexpected end-of-file");
@@ -39,8 +61,15 @@ void writeFull(int fd, std::string_view s, bool allowInterrupts)
     while (!s.empty()) {
         if (allowInterrupts) checkInterrupt();
         ssize_t res = write(fd, s.data(), s.size());
-        if (res == -1 && errno != EINTR)
+        if (res == -1) {
+            switch (errno) {
+            case EINTR: continue;
+            case EAGAIN:
+                pollFD(fd, POLLOUT);
+                continue;
+            }
             throw SysError("writing to file");
+        }
         if (res > 0)
             s.remove_prefix(res);
     }
@@ -56,8 +85,15 @@ std::string readLine(int fd, bool eofOk)
         // FIXME: inefficient
         ssize_t rd = read(fd, &ch, 1);
         if (rd == -1) {
-            if (errno != EINTR)
+            switch (errno) {
+            case EINTR: continue;
+            case EAGAIN: {
+                pollFD(fd, POLLIN);
+                continue;
+            }
+            default:
                 throw SysError("reading a line");
+            }
         } else if (rd == 0) {
             if (eofOk)
                 return s;