* In `nix-env -i|-u|-e', lock the profile to prevent races between

concurrent nix-env operations on the same profile.  Fixes NIX-7.

src/libstore/pathlocks.cc

@@ -51,14 +51,14 @@ PathLocks::PathLocks()
 }
 
 
-PathLocks::PathLocks(const PathSet & paths)
+PathLocks::PathLocks(const PathSet & paths, const string & waitMsg)
     : deletePaths(false)
 {
-    lockPaths(paths);
+    lockPaths(paths, waitMsg);
 }
 
 
-void PathLocks::lockPaths(const PathSet & _paths)
+void PathLocks::lockPaths(const PathSet & _paths, const string & waitMsg)
 {
     /* May be called only once! */
     assert(fds.empty());
@@ -94,7 +94,10 @@ void PathLocks::lockPaths(const PathSet & _paths)
                 throw SysError(format("opening lock file `%1%'") % lockPath);
 
             /* Acquire an exclusive lock. */
-            lockFile(fd, ltWrite, true);
+            if (!lockFile(fd, ltWrite, false)) {
+                if (waitMsg != "") printMsg(lvlError, waitMsg);
+                lockFile(fd, ltWrite, true);
+            }
 
             debug(format("lock acquired on `%1%'") % lockPath);
 

src/libstore/pathlocks.hh

@@ -18,8 +18,10 @@ private:
 
 public:
     PathLocks();
-    PathLocks(const PathSet & paths);
-    void lockPaths(const PathSet & _paths);
+    PathLocks(const PathSet & paths,
+        const string & waitMsg = "");
+    void lockPaths(const PathSet & _paths,
+        const string & waitMsg = "");
     ~PathLocks();
     void setDeletion(bool deletePaths);
 };

src/nix-env/main.cc

@@ -10,6 +10,7 @@
 #include "help.txt.hh"
 #include "nixexpr-ast.hh"
 #include "get-drvs.hh"
+#include "pathlocks.hh"
 
 #include <cerrno>
 #include <ctime>
@@ -411,6 +412,14 @@ static void printMissing(EvalState & state, const DrvInfos & elems)
 }
 
 
+static void lockProfile(PathLocks & lock, const Path & profile)
+{
+    lock.lockPaths(singleton<PathSet>(profile),
+        (format("waiting for lock on profile `%1%'") % profile).str());
+    lock.setDeletion(true);
+}
+
+
 static void installDerivations(Globals & globals,
     const Strings & args, const Path & profile)
 {
@@ -426,6 +435,8 @@ static void installDerivations(Globals & globals,
 
     /* Add in the already installed derivations, unless they have the
        same name as a to-be-installed element. */
+    PathLocks lock;
+    lockProfile(lock, profile);
     DrvInfos installedElems = queryInstalled(globals.state, profile);
 
     DrvInfos allElems(newElems);
@@ -480,6 +491,8 @@ static void upgradeDerivations(Globals & globals,
        name and a higher version number. */
 
     /* Load the currently installed derivations. */
+    PathLocks lock;
+    lockProfile(lock, profile);
     DrvInfos installedElems = queryInstalled(globals.state, profile);
 
     /* Fetch all derivations from the input file. */
@@ -559,6 +572,8 @@ static void opUpgrade(Globals & globals,
 static void uninstallDerivations(Globals & globals, DrvNames & selectors,
     Path & profile)
 {
+    PathLocks lock;
+    lockProfile(lock, profile);
     DrvInfos installedElems = queryInstalled(globals.state, profile);
     DrvInfos newElems;