* Use optimistic profile locking for nix-env operations like `-i' and

`-u'.  Instead of acquiring an exclusive lock on the profile for the
  entire duration of the operation, we just perform the operation
  optimistically (without an exclusive lock), and check at the end
  whether the profile changed while we were busy (i.e., the symlink
  target changed).  If so, the operation is restarted.  Restarting is
  generally cheap, since the build results are still in the Nix store.
  Most of the time, only the user environment has to be rebuilt.

src/nix-env/nix-env.cc

@@ -215,8 +215,34 @@ static DrvInfos queryInstalled(EvalState & state, const Path & userEnv)
 }
 
 
-static void createUserEnv(EvalState & state, const DrvInfos & elems,
-    const Path & profile, bool keepDerivations)
+/* Ensure exclusive access to a profile.  Any command that modifies
+   the profile first acquires this lock. */
+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);
+}
+
+
+/* Optimistic locking is used by long-running operations like `nix-env
+   -i'.  Instead of acquiring the exclusive lock for the entire
+   duration of the operation, we just perform the operation
+   optimistically (without an exclusive lock), and check at the end
+   whether the profile changed while we were busy (i.e., the symlink
+   target changed).  If so, the operation is restarted.  Restarting is
+   generally cheap, since the build results are still in the Nix
+   store.  Most of the time, only the user environment has to be
+   rebuilt. */
+static string optimisticLockProfile(const Path & profile)
+{
+    return pathExists(profile) ? readLink(profile) : "";
+}
+
+
+static bool createUserEnv(EvalState & state, const DrvInfos & elems,
+    const Path & profile, bool keepDerivations,
+    const string & lockToken)
 {
     /* Build the components in the user environment, if they don't
        exist already. */
@@ -305,9 +331,20 @@ static void createUserEnv(EvalState & state, const DrvInfos & elems,
     store->buildDerivations(singleton<PathSet>(topLevelDrv.queryDrvPath(state)));
 
     /* Switch the current user environment to the output path. */
+    PathLocks lock;
+    lockProfile(lock, profile);
+
+    Path lockTokenCur = optimisticLockProfile(profile);
+    if (lockToken != lockTokenCur) {
+        printMsg(lvlError, format("profile `%1%' changed while we were busy; restarting") % profile);
+        return false;
+    }
+    
     debug(format("switching to new user environment"));
     Path generation = createGeneration(profile, topLevelDrv.queryOutPath(state));
     switchLink(profile, generation);
+
+    return true;
 }
 
 
@@ -544,14 +581,6 @@ 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)
 {
@@ -574,35 +603,35 @@ 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);
-    for (DrvInfos::iterator i = installedElems.begin();
-         i != installedElems.end(); ++i)
-    {
-        DrvName drvName(i->name);
-        MetaInfo meta = i->queryMetaInfo(globals.state);
-        if (!globals.preserveInstalled &&
-            newNames.find(drvName.name) != newNames.end() &&
-            meta["keep"] != "true")
-            printMsg(lvlInfo,
-                format("replacing old `%1%'") % i->name);
-        else
-            allElems.push_back(*i);
+    while (true) {
+        string lockToken = optimisticLockProfile(profile);
+        
+        DrvInfos installedElems = queryInstalled(globals.state, profile);
+        
+        DrvInfos allElems(newElems);
+        foreach (DrvInfos::iterator, i, installedElems) {
+            DrvName drvName(i->name);
+            MetaInfo meta = i->queryMetaInfo(globals.state);
+            if (!globals.preserveInstalled &&
+                newNames.find(drvName.name) != newNames.end() &&
+                meta["keep"] != "true")
+                printMsg(lvlInfo,
+                    format("replacing old `%1%'") % i->name);
+            else
+                allElems.push_back(*i);
+        }
+
+        foreach (DrvInfos::iterator, i, newElems)
+            printMsg(lvlInfo, format("installing `%1%'") % i->name);
+    
+        printMissing(globals.state, newElems);
+    
+        if (globals.dryRun) return;
+
+        if (createUserEnv(globals.state, allElems,
+                profile, globals.keepDerivations, lockToken)) break;
     }
-
-    for (DrvInfos::iterator i = newElems.begin(); i != newElems.end(); ++i)
-        printMsg(lvlInfo,
-            format("installing `%1%'") % i->name);
-    
-    printMissing(globals.state, newElems);
-    
-    if (globals.dryRun) return;
-
-    createUserEnv(globals.state, allElems,
-        profile, globals.keepDerivations);
 }
 
 
@@ -634,77 +663,75 @@ static void upgradeDerivations(Globals & globals,
        for a derivation in the input Nix expression that has the same
        name and a higher version number. */
 
-    /* Load the currently installed derivations. */
-    PathLocks lock;
-    lockProfile(lock, globals.profile);
-    DrvInfos installedElems = queryInstalled(globals.state, globals.profile);
+    while (true) {
+        string lockToken = optimisticLockProfile(globals.profile);
+        
+        DrvInfos installedElems = queryInstalled(globals.state, globals.profile);
 
-    /* Fetch all derivations from the input file. */
-    DrvInfos availElems;
-    queryInstSources(globals.state, globals.instSource, args, availElems, false);
+        /* Fetch all derivations from the input file. */
+        DrvInfos availElems;
+        queryInstSources(globals.state, globals.instSource, args, availElems, false);
 
-    /* Go through all installed derivations. */
-    DrvInfos newElems;
-    for (DrvInfos::iterator i = installedElems.begin();
-         i != installedElems.end(); ++i)
-    {
-        DrvName drvName(i->name);
+        /* Go through all installed derivations. */
+        DrvInfos newElems;
+        foreach (DrvInfos::iterator, i, installedElems) {
+            DrvName drvName(i->name);
 
-        MetaInfo meta = i->queryMetaInfo(globals.state);
-        if (meta["keep"] == "true") {
-            newElems.push_back(*i);
-            continue;
-        }
+            MetaInfo meta = i->queryMetaInfo(globals.state);
+            if (meta["keep"] == "true") {
+                newElems.push_back(*i);
+                continue;
+            }
 
-        /* Find the derivation in the input Nix expression with the
-           same name that satisfies the version constraints specified
-           by upgradeType.  If there are multiple matches, take the
-           one with the highest priority.  If there are still multiple
-           matches, take the one with the highest version. */
-        DrvInfos::iterator bestElem = availElems.end();
-        DrvName bestName;
-        for (DrvInfos::iterator j = availElems.begin();
-             j != availElems.end(); ++j)
-        {
-            DrvName newName(j->name);
-            if (newName.name == drvName.name) {
-                int d = comparePriorities(globals.state, *i, *j);
-                if (d == 0) d = compareVersions(drvName.version, newName.version);
-                if (upgradeType == utLt && d < 0 ||
-                    upgradeType == utLeq && d <= 0 ||
-                    upgradeType == utEq && d == 0 ||
-                    upgradeType == utAlways)
-                {
-                    int d2 = -1;
-                    if (bestElem != availElems.end()) {
-                        d2 = comparePriorities(globals.state, *bestElem, *j);
-                        if (d2 == 0) d2 = compareVersions(bestName.version, newName.version);
-                    }
-                    if (d2 < 0) {
-                        bestElem = j;
-                        bestName = newName;
+            /* Find the derivation in the input Nix expression with
+               the same name that satisfies the version constraints
+               specified by upgradeType.  If there are multiple
+               matches, take the one with the highest priority.  If
+               there are still multiple matches, take the one with the
+               highest version. */
+            DrvInfos::iterator bestElem = availElems.end();
+            DrvName bestName;
+            foreach (DrvInfos::iterator, j, availElems) {
+                DrvName newName(j->name);
+                if (newName.name == drvName.name) {
+                    int d = comparePriorities(globals.state, *i, *j);
+                    if (d == 0) d = compareVersions(drvName.version, newName.version);
+                    if (upgradeType == utLt && d < 0 ||
+                        upgradeType == utLeq && d <= 0 ||
+                        upgradeType == utEq && d == 0 ||
+                        upgradeType == utAlways)
+                    {
+                        int d2 = -1;
+                        if (bestElem != availElems.end()) {
+                            d2 = comparePriorities(globals.state, *bestElem, *j);
+                            if (d2 == 0) d2 = compareVersions(bestName.version, newName.version);
+                        }
+                        if (d2 < 0) {
+                            bestElem = j;
+                            bestName = newName;
+                        }
                     }
                 }
             }
-        }
 
-        if (bestElem != availElems.end() &&
-            i->queryOutPath(globals.state) !=
+            if (bestElem != availElems.end() &&
+                i->queryOutPath(globals.state) !=
                 bestElem->queryOutPath(globals.state))
-        {
-            printMsg(lvlInfo,
-                format("upgrading `%1%' to `%2%'")
-                % i->name % bestElem->name);
-            newElems.push_back(*bestElem);
-        } else newElems.push_back(*i);
-    }
+            {
+                printMsg(lvlInfo,
+                    format("upgrading `%1%' to `%2%'")
+                    % i->name % bestElem->name);
+                newElems.push_back(*bestElem);
+            } else newElems.push_back(*i);
+        }
     
-    printMissing(globals.state, newElems);
+        printMissing(globals.state, newElems);
     
-    if (globals.dryRun) return;
+        if (globals.dryRun) return;
 
-    createUserEnv(globals.state, newElems,
-        globals.profile, globals.keepDerivations);
+        if (createUserEnv(globals.state, newElems,
+                globals.profile, globals.keepDerivations, lockToken)) break;
+    }
 }
 
 
@@ -748,29 +775,27 @@ static void opSetFlag(Globals & globals,
     string flagValue = *arg++;
     DrvNames selectors = drvNamesFromArgs(Strings(arg, opArgs.end()));
 
-    /* Load the currently installed derivations. */
-    PathLocks lock;
-    lockProfile(lock, globals.profile);
-    DrvInfos installedElems = queryInstalled(globals.state, globals.profile);
+    while (true) {
+        string lockToken = optimisticLockProfile(globals.profile);
 
-    /* Update all matching derivations. */
-    for (DrvInfos::iterator i = installedElems.begin();
-         i != installedElems.end(); ++i)
-    {
-        DrvName drvName(i->name);
-        for (DrvNames::iterator j = selectors.begin();
-             j != selectors.end(); ++j)
-            if (j->matches(drvName)) {
-                printMsg(lvlInfo,
-                    format("setting flag on `%1%'") % i->name);
-                setMetaFlag(globals.state, *i, flagName, flagValue);
-                break;
-            }
+        DrvInfos installedElems = queryInstalled(globals.state, globals.profile);
+
+        /* Update all matching derivations. */
+        foreach (DrvInfos::iterator, i, installedElems) {
+            DrvName drvName(i->name);
+            foreach (DrvNames::iterator, j, selectors)
+                if (j->matches(drvName)) {
+                    printMsg(lvlInfo,
+                        format("setting flag on `%1%'") % i->name);
+                    setMetaFlag(globals.state, *i, flagName, flagValue);
+                    break;
+                }
+        }
+
+        /* Write the new user environment. */
+        if (createUserEnv(globals.state, installedElems,
+                globals.profile, globals.keepDerivations, lockToken)) break;
     }
-
-    /* Write the new user environment. */
-    createUserEnv(globals.state, installedElems,
-        globals.profile, globals.keepDerivations);
 }
 
 
@@ -812,33 +837,33 @@ static void opSet(Globals & globals,
 static void uninstallDerivations(Globals & globals, Strings & selectors,
     Path & profile)
 {
-    PathLocks lock;
-    lockProfile(lock, profile);
-    DrvInfos installedElems = queryInstalled(globals.state, profile);
-    DrvInfos newElems;
+    while (true) {
+        string lockToken = optimisticLockProfile(profile);
 
-    for (DrvInfos::iterator i = installedElems.begin();
-         i != installedElems.end(); ++i)
-    {
-        DrvName drvName(i->name);
-        bool found = false;
-        for (Strings::iterator j = selectors.begin(); j != selectors.end(); ++j)
-            /* !!! the repeated calls to followLinksToStorePath() are
-               expensive, should pre-compute them. */
-            if ((isPath(*j) && i->queryOutPath(globals.state) == followLinksToStorePath(*j))
-                || DrvName(*j).matches(drvName))
-            {
-                printMsg(lvlInfo, format("uninstalling `%1%'") % i->name);
-                found = true;
-                break;
-            }
-        if (!found) newElems.push_back(*i);
+        DrvInfos installedElems = queryInstalled(globals.state, profile);
+        DrvInfos newElems;
+
+        foreach (DrvInfos::iterator, i, installedElems) {
+            DrvName drvName(i->name);
+            bool found = false;
+            foreach (Strings::iterator, j, selectors)
+                /* !!! the repeated calls to followLinksToStorePath()
+                   are expensive, should pre-compute them. */
+                if ((isPath(*j) && i->queryOutPath(globals.state) == followLinksToStorePath(*j))
+                    || DrvName(*j).matches(drvName))
+                {
+                    printMsg(lvlInfo, format("uninstalling `%1%'") % i->name);
+                    found = true;
+                    break;
+                }
+            if (!found) newElems.push_back(*i);
+        }
+
+        if (globals.dryRun) return;
+
+        if (createUserEnv(globals.state, newElems,
+                profile, globals.keepDerivations, lockToken)) break;
     }
-
-    if (globals.dryRun) return;
-
-    createUserEnv(globals.state, newElems,
-        profile, globals.keepDerivations);
 }