* Concurrent GC on Cygwin.

src/libstore/gc.cc

@@ -11,6 +11,11 @@
 #include <fcntl.h>
 #include <unistd.h>
 
+#ifdef __CYGWIN__
+#include <windows.h>
+#include <sys/cygwin.h>
+#endif
+
 
 static string gcLockName = "gc.lock";
 static string tempRootsDir = "temproots";
@@ -108,10 +113,6 @@ static AutoCloseFD fdTempRoots;
 
 void addTempRoot(const Path & path)
 {
-#ifdef __CYGWIN__
-    return;
-#endif	
-    
     /* Create the temporary roots file for this process. */
     if (fdTempRoots == -1) {
 
@@ -124,12 +125,24 @@ void addTempRoot(const Path & path)
 
             AutoCloseFD fdGCLock = openGCLock(ltRead);
             
-            fdTempRoots = open(fnTempRoots.c_str(), O_RDWR | O_CREAT | O_TRUNC, 0600);
+            if (pathExists(fnTempRoots))
-            if (fdTempRoots == -1)
+                /* It *must* be stale, since there can be no two
-                throw SysError(format("opening temporary roots file `%1%'") % fnTempRoots);
+                   processes with the same pid. */
+                deletePath(fnTempRoots);
+
+	    fdTempRoots = openLockFile(fnTempRoots, true);
 
             fdGCLock.close();
-            
+      
+	    /* Note that on Cygwin a lot of the following complexity
+	       is unnecessary, since we cannot delete open lock
+	       files.  If we have the lock file open, then it's valid;
+	       if we can delete it, then it wasn't in use any more. 
+
+	       Also note that on Cygwin we cannot "upgrade" a lock
+	       from a read lock to a write lock. */
+
+#ifndef __CYGWIN__
             debug(format("acquiring read lock on `%1%'") % fnTempRoots);
             lockFile(fdTempRoots, ltRead, true);
 
@@ -143,6 +156,10 @@ void addTempRoot(const Path & path)
             /* The garbage collector deleted this file before we could
                get a lock.  (It won't delete the file after we get a
                lock.)  Try again. */
+
+#else
+            break;
+#endif
         }
 
     }
@@ -155,9 +172,14 @@ void addTempRoot(const Path & path)
     string s = path + '\0';
     writeFull(fdTempRoots, (const unsigned char *) s.c_str(), s.size());
 
+#ifndef __CYGWIN__
     /* Downgrade to a read lock. */
     debug(format("downgrading to read lock on `%1%'") % fnTempRoots);
     lockFile(fdTempRoots, ltRead, true);
+#else
+    debug(format("releasing write lock on `%1%'") % fnTempRoots);
+    lockFile(fdTempRoots, ltNone, true);
+#endif
 }
 
 
@@ -176,10 +198,6 @@ typedef list<FDPtr> FDs;
 
 static void readTempRoots(PathSet & tempRoots, FDs & fds)
 {
-#ifdef __CYGWIN__
-    return;
-#endif
-    
     /* Read the `temproots' directory for per-process temporary root
        files. */
     Strings tempRootFiles = readDirectory(
@@ -191,7 +209,19 @@ static void readTempRoots(PathSet & tempRoots, FDs & fds)
         Path path = (format("%1%/%2%/%3%") % nixStateDir % tempRootsDir % *i).str();
 
         debug(format("reading temporary root file `%1%'") % path);
-            
+
+#ifdef __CYGWIN__
+	/* On Cygwin we just try to delete the lock file. */
+	char win32Path[MAX_PATH];
+	cygwin_conv_to_full_win32_path(path.c_str(), win32Path);
+	if (DeleteFile(win32Path)) {
+            printMsg(lvlError, format("removed stale temporary roots file `%1%'")
+                % path);
+            continue;
+        } else
+            debug(format("delete of `%1%' failed: %2%") % path % GetLastError());
+#endif
+
         FDPtr fd(new AutoCloseFD(open(path.c_str(), O_RDWR, 0666)));
         if (*fd == -1) {
             /* It's okay if the file has disappeared. */
@@ -199,6 +229,11 @@ static void readTempRoots(PathSet & tempRoots, FDs & fds)
             throw SysError(format("opening temporary roots file `%1%'") % path);
         }
 
+        /* This should work, but doesn't, for some reason. */
+        //FDPtr fd(new AutoCloseFD(openLockFile(path, false)));
+        //if (*fd == -1) continue;
+
+#ifndef __CYGWIN__
         /* Try to acquire a write lock without blocking.  This can
            only succeed if the owning process has died.  In that case
            we don't care about its temporary roots. */
@@ -209,6 +244,7 @@ static void readTempRoots(PathSet & tempRoots, FDs & fds)
             writeFull(*fd, (const unsigned char *) "d", 1);
             continue;
         }
+#endif
 
         /* Acquire a read lock.  This will prevent the owning process
            from upgrading to a write lock, therefore it will block in
@@ -448,28 +484,24 @@ void collectGarbage(GCAction action, const PathSet & pathsToDelete,
         debug(format("dead path `%1%'") % *i);
         result.insert(*i);
 
-        AutoCloseFD fdLock;
-
         if (action == gcDeleteDead || action == gcDeleteSpecific) {
 
+#ifndef __CYGWIN__
+            AutoCloseFD fdLock;
+
             /* Only delete a lock file if we can acquire a write lock
                on it.  That means that it's either stale, or the
                process that created it hasn't locked it yet.  In the
                latter case the other process will detect that we
                deleted the lock, and retry (see pathlocks.cc). */
             if (i->size() >= 5 && string(*i, i->size() - 5) == ".lock") {
-
+                fdLock = openLockFile(*i, false);
-                fdLock = open(i->c_str(), O_RDWR);
+                if (fdLock != -1 && !lockFile(fdLock, ltWrite, false)) {
-                if (fdLock == -1) {
-                    if (errno == ENOENT) continue;
-                    throw SysError(format("opening lock file `%1%'") % *i);
-                }
-
-                if (!lockFile(fdLock, ltWrite, false)) {
                     debug(format("skipping active lock `%1%'") % *i);
                     continue;
                 }
             }
+#endif
 
             printMsg(lvlInfo, format("deleting `%1%'") % *i);
             
@@ -478,9 +510,11 @@ void collectGarbage(GCAction action, const PathSet & pathsToDelete,
             deleteFromStore(*i, freed);
             bytesFreed += freed;
 
+#ifndef __CYGWIN__
             if (fdLock != -1)
                 /* Write token to stale (deleted) lock file. */
                 writeFull(fdLock, (const unsigned char *) "d", 1);
+#endif
         }
     }
 }

src/libstore/pathlocks.cc

@@ -12,6 +12,79 @@
 #endif
 
 
+int openLockFile(const Path & path, bool create)
+{
+    AutoCloseFD fd;
+
+#ifdef __CYGWIN__
+    /* On Cygwin we have to open the lock file without "DELETE"
+       sharing mode; otherwise Windows will allow open lock files to
+       be deleted (which is almost but not quite what Unix does). */
+    char win32Path[MAX_PATH + 1];
+    cygwin_conv_to_full_win32_path(path.c_str(), win32Path);
+
+    SECURITY_ATTRIBUTES sa; /* required, otherwise inexplicably bad shit happens */
+    sa.nLength = sizeof sa;
+    sa.lpSecurityDescriptor = 0;
+    sa.bInheritHandle = TRUE;
+    HANDLE h = CreateFile(win32Path, GENERIC_READ | GENERIC_WRITE,
+        FILE_SHARE_READ | FILE_SHARE_WRITE, &sa, 
+        (create ? OPEN_ALWAYS : OPEN_EXISTING), 
+        FILE_ATTRIBUTE_NORMAL, 0);
+    if (h == INVALID_HANDLE_VALUE) {
+        if (create || GetLastError() != ERROR_FILE_NOT_FOUND)
+            throw Error(format("opening lock file `%1%'") % path);
+        fd = -1;
+    }
+    else
+        fd = cygwin_attach_handle_to_fd((char *) path.c_str(), -1, h, 1, O_RDWR);
+#else        
+    fd = open(path.c_str(), O_RDWR | (create ? O_CREAT : 0), 0666);
+    if (fd == -1 && (create || errno != ENOENT))
+        throw SysError(format("opening lock file `%1%'") % path);
+#endif
+
+    return fd.borrow();
+}
+
+
+void deleteLockFilePreClose(const Path & path, int fd)
+{
+#ifndef __CYGWIN__
+    /* Get rid of the lock file.  Have to be careful not to introduce
+       races. */
+    /* On Unix, write a (meaningless) token to the file to indicate to
+       other processes waiting on this lock that the lock is stale
+       (deleted). */
+    unlink(path.c_str());
+    writeFull(fd, (const unsigned char *) "d", 1);
+    /* Note that the result of unlink() is ignored; removing the lock
+       file is an optimisation, not a necessity. */
+#endif
+}
+
+
+void deleteLockFilePostClose(const Path & path)
+{
+#ifdef __CYGWIN__
+    /* On Windows, just try to delete the lock file.  This will fail
+       if anybody still has the file open.  We cannot use unlink()
+       here, because Cygwin emulates Unix semantics of allowing an
+       open file to be deleted (but fakes it - the file isn't actually
+       deleted until later, so a file with the same name cannot be
+       created in the meantime). */
+    char win32Path[MAX_PATH + 1];
+    cygwin_conv_to_full_win32_path(path.c_str(), win32Path);
+    if (DeleteFile(win32Path))
+        debug(format("delete of `%1%' succeeded") % path.c_str());
+    else
+        /* Not an error: probably means that the lock is still opened
+           by someone else. */
+        debug(format("delete of `%1%' failed: %2%") % path.c_str() % GetLastError());
+#endif
+}
+
+
 bool lockFile(int fd, LockType lockType, bool wait)
 {
     struct flock lock;
@@ -94,26 +167,7 @@ void PathLocks::lockPaths(const PathSet & _paths, const string & waitMsg)
         while (1) {
 
             /* Open/create the lock file. */
-#ifdef __CYGWIN__
+	    fd = openLockFile(lockPath, true);
-	    char win32Path[MAX_PATH];
-	    cygwin_conv_to_full_win32_path(lockPath.c_str(), win32Path);
-
-	    SECURITY_ATTRIBUTES sa;
-	    sa.nLength = sizeof sa;
-	    sa.lpSecurityDescriptor = 0;
-	    sa.bInheritHandle = TRUE;
-	    HANDLE h = CreateFile(win32Path, GENERIC_READ, 
-	        FILE_SHARE_READ | FILE_SHARE_WRITE, &sa, OPEN_ALWAYS, 
-                FILE_ATTRIBUTE_NORMAL, 0);
-	    if (h == INVALID_HANDLE_VALUE)
-		throw Error(format("opening lock file `%1%'") % lockPath);
-
-	    fd = cygwin_attach_handle_to_fd((char *) lockPath.c_str(), -1, h, 1, O_RDWR);
-#else        
-            fd = open(lockPath.c_str(), O_WRONLY | O_CREAT, 0666);
-            if (fd == -1)
-                throw SysError(format("opening lock file `%1%'") % lockPath);
-#endif
 
             /* Acquire an exclusive lock. */
             if (!lockFile(fd, ltWrite, false)) {
@@ -148,40 +202,15 @@ void PathLocks::lockPaths(const PathSet & _paths, const string & waitMsg)
 PathLocks::~PathLocks()
 {
     for (list<FDPair>::iterator i = fds.begin(); i != fds.end(); i++) {
-#ifndef __CYGWIN__
+        if (deletePaths) deleteLockFilePreClose(i->second, i->first);
-        if (deletePaths) {
+
-	    /* Get rid of the lock file.  Have to be careful not to
-	       introduce races. */
-            /* On Unix, write a (meaningless) token to the file to
-               indicate to other processes waiting on this lock that
-               the lock is stale (deleted). */
-            unlink(i->second.c_str());
-            writeFull(i->first, (const unsigned char *) "d", 1);
-            /* Note that the result of unlink() is ignored; removing
-               the lock file is an optimisation, not a necessity. */
-        }
-#endif
         lockedPaths.erase(i->second);
         if (close(i->first) == -1)
             printMsg(lvlError,
                 format("error (ignored): cannot close lock file on `%1%'") % i->second);
-#ifdef __CYGWIN__
+
-	if (deletePaths) {
+	if (deletePaths) deleteLockFilePostClose(i->second);
-	    /* On Windows, just try to delete the lock file.  This
+
-	       will fail if anybody still has the file open.  We
-	       cannot use unlink() here, because Cygwin emulates Unix
-	       semantics of allowing an open file to be deleted (but
-	       fakes it - the file isn't actually deleted until later,
-	       so a file with the same name cannot be created in the
-	       meantime). */
-	    char win32Path[MAX_PATH];
-	    cygwin_conv_to_full_win32_path(i->second.c_str(), win32Path);
-	    if (DeleteFile(win32Path))
-		debug(format("delete of `%1%' succeeded") % i->second.c_str());
-	    else
-		debug(format("delete of `%1%' failed: %2%") % i->second.c_str() % GetLastError());
-	}
-#endif
         debug(format("lock released on `%1%'") % i->second);
     }
 }

src/libstore/pathlocks.hh

@@ -4,6 +4,16 @@
 #include "util.hh"
 
 
+/* Open (possibly create) a lock file and return the file descriptor.
+   -1 is returned if create is false and the lock could not be opened
+   because it doesn't exist.  Any other error throws an exception. */
+int openLockFile(const Path & path, bool create);
+
+/* Delete an open lock file.  Both must be called to be fully portable
+   between Unix and Windows. */
+void deleteLockFilePreClose(const Path & path, int fd);
+void deleteLockFilePostClose(const Path & path);
+
 typedef enum LockType { ltRead, ltWrite, ltNone };
 
 bool lockFile(int fd, LockType lockType, bool wait);