LocalStore::addTempRoot(): Handle ENOENT

src/libstore/gc.cc

@@ -149,11 +149,12 @@ void LocalStore::addTempRoot(const StorePath & path)
             try {
                 nix::connect(fdRootsSocket->get(), socketPath);
             } catch (SysError & e) {
-                /* The garbage collector may have exited, so we need to
-                   restart. */
-                if (e.errNo == ECONNREFUSED) {
-                    debug("GC socket connection refused");
+                /* The garbage collector may have exited or not
+                   created the socket yet, so we need to restart. */
+                if (e.errNo == ECONNREFUSED || e.errNo == ENOENT) {
+                    debug("GC socket connection refused: %s", e.msg());
                     fdRootsSocket->close();
+                    std::this_thread::sleep_for(std::chrono::milliseconds(100));
                     goto restart;
                 }
                 throw;
@@ -509,6 +510,11 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
     auto fdGCLock = openGCLock();
     FdLock gcLock(fdGCLock.get(), ltWrite, true, "waiting for the big garbage collector lock...");
 
+    /* Synchronisation point to test ENOENT handling in
+       addTempRoot(), see tests/gc-non-blocking.sh. */
+    if (auto p = getEnv("_NIX_TEST_GC_SYNC_1"))
+        readFile(*p);
+
     /* Start the server for receiving new roots. */
     auto socketPath = stateDir.get() + gcSocketPath;
     createDirs(dirOf(socketPath));
@@ -632,6 +638,10 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
         roots.insert(root.first);
     }
 
+    /* Synchronisation point for testing, see tests/functional/gc-non-blocking.sh. */
+    if (auto p = getEnv("_NIX_TEST_GC_SYNC_2"))
+        readFile(*p);
+
     /* Helper function that deletes a path from the store and throws
        GCLimitReached if we've deleted enough garbage. */
     auto deleteFromStore = [&](std::string_view baseName)
@@ -778,10 +788,6 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
         }
     };
 
-    /* Synchronisation point for testing, see tests/functional/gc-concurrent.sh. */
-    if (auto p = getEnv("_NIX_TEST_GC_SYNC"))
-        readFile(*p);
-
     /* Either delete all garbage paths, or just the specified
        paths (for gcDeleteSpecific). */
     if (options.action == GCOptions::gcDeleteSpecific) {

src/libutil/unix-domain-socket.cc

@@ -1,6 +1,7 @@
 #include "file-system.hh"
 #include "processes.hh"
 #include "unix-domain-socket.hh"
+#include "util.hh"
 
 #include <sys/socket.h>
 #include <sys/un.h>
@@ -75,21 +76,35 @@ void connect(int fd, const std::string & path)
     addr.sun_family = AF_UNIX;
 
     if (path.size() + 1 >= sizeof(addr.sun_path)) {
+        Pipe pipe;
+        pipe.create();
         Pid pid = startProcess([&]() {
-            Path dir = dirOf(path);
-            if (chdir(dir.c_str()) == -1)
-                throw SysError("chdir to '%s' failed", dir);
-            std::string base(baseNameOf(path));
-            if (base.size() + 1 >= sizeof(addr.sun_path))
-                throw Error("socket path '%s' is too long", base);
-            memcpy(addr.sun_path, base.c_str(), base.size() + 1);
-            if (connect(fd, (struct sockaddr *) &addr, sizeof(addr)) == -1)
-                throw SysError("cannot connect to socket at '%s'", path);
-            _exit(0);
+            try {
+                pipe.readSide.close();
+                Path dir = dirOf(path);
+                if (chdir(dir.c_str()) == -1)
+                    throw SysError("chdir to '%s' failed", dir);
+                std::string base(baseNameOf(path));
+                if (base.size() + 1 >= sizeof(addr.sun_path))
+                    throw Error("socket path '%s' is too long", base);
+                memcpy(addr.sun_path, base.c_str(), base.size() + 1);
+                if (connect(fd, (struct sockaddr *) &addr, sizeof(addr)) == -1)
+                    throw SysError("cannot connect to socket at '%s'", path);
+                writeFull(pipe.writeSide.get(), "0\n");
+            } catch (SysError & e) {
+                writeFull(pipe.writeSide.get(), fmt("%d\n", e.errNo));
+            } catch (...) {
+                writeFull(pipe.writeSide.get(), "-1\n");
+            }
         });
-        int status = pid.wait();
-        if (status != 0)
+        pipe.writeSide.close();
+        auto errNo = string2Int<int>(chomp(drainFD(pipe.readSide.get())));
+        if (!errNo || *errNo == -1)
             throw Error("cannot connect to socket at '%s'", path);
+        else if (*errNo > 0) {
+            errno = *errNo;
+            throw SysError("cannot connect to socket at '%s'", path);
+        }
     } else {
         memcpy(addr.sun_path, path.c_str(), path.size() + 1);
         if (connect(fd, (struct sockaddr *) &addr, sizeof(addr)) == -1)

tests/functional/gc-non-blocking.sh

@@ -6,27 +6,42 @@ needLocalStore "the GC test needs a synchronisation point"
 
 clearStore
 
-fifo=$TEST_ROOT/test.fifo
-mkfifo "$fifo"
+# This FIFO is read just after the global GC lock has been acquired,
+# but before the root server is started.
+fifo1=$TEST_ROOT/test2.fifo
+mkfifo "$fifo1"
+
+# This FIFO is read just after the roots have been read, but before
+# the actual GC starts.
+fifo2=$TEST_ROOT/test.fifo
+mkfifo "$fifo2"
 
 dummy=$(nix store add-path ./simple.nix)
 
 running=$TEST_ROOT/running
 touch $running
 
-(_NIX_TEST_GC_SYNC=$fifo nix-store --gc -vvvvv; rm $running) &
+# Start GC.
+(_NIX_TEST_GC_SYNC_1=$fifo1 _NIX_TEST_GC_SYNC_2=$fifo2 nix-store --gc -vvvvv; rm $running) &
 pid=$!
 
 sleep 2
 
+# Delay the start of the root server to check that the build below
+# correctly handles ENOENT when connecting to the root server.
+(sleep 1; echo > $fifo1) &
+pid2=$!
+
+# Start a build. This should not be blocked by the GC in progress.
 outPath=$(nix-build --max-silent-time 60 -o "$TEST_ROOT/result" -E "
   with import ./config.nix;
   mkDerivation {
     name = \"non-blocking\";
-    buildCommand = \"set -x; test -e $running; mkdir \$out; echo > $fifo\";
+    buildCommand = \"set -x; test -e $running; mkdir \$out; echo > $fifo2\";
   }")
 
 wait $pid
+wait $pid2
 
 (! test -e $running)
 (! test -e $dummy)