Update to semaphore-compat 2.0.0 fixing #9993 and https://gitlab.haskell.org/ghc/ghc/-/issues/25087

semaphore-compat now uses a unix sockets based implementation.

Semaphore identifiers are now versioned, and we can really on the versioning
scheme to detect semaphore version mismatch with GHC and fallback gracefully if
possible.

GHC patch: https://gitlab.haskell.org/ghc/ghc/-/merge_requests/15729

After the GHC patch lands, it will include a "Semaphore version" field in its
settings file/`--info` output that we can use to guide Cabal behaviour.

If this field does not exist (and ghc is 9.8+), then we assume it uses version v1 of the protocol
and this triggers a graceful degradation of behaviour to `-jN` without semaphore based coordination.

See also

semaphore-compat MR: https://gitlab.haskell.org/ghc/semaphore-compat/-/merge_requests/8

ghc-proposals change: ghc-proposals/ghc-proposals#673

Author: wz1000

Cabal/src/Distribution/Simple/Compiler.hs

@@ -86,6 +86,7 @@ module Distribution.Simple.Compiler
   , libraryDynDirSupported
   , libraryVisibilitySupported
   , jsemSupported
+  , jsemVersion
   , reexportedAsSupported
 
     -- * Support for profiling detail levels
@@ -480,6 +481,17 @@ jsemSupported comp = case compilerFlavor comp of
   where
     v = compilerVersion comp
 
+-- | What semaphore protocol version does this compiler use?
+--
+-- Returns @Nothing@ for compilers that don't report a "Semaphore version"
+-- field in @ghc --info@ (i.e. GHC 9.8–9.14, which use v1).
+jsemVersion :: Compiler -> Maybe Int
+jsemVersion comp = case compilerFlavor comp of
+  GHC -> case Map.lookup "Semaphore version" (compilerProperties comp) of
+    Just verStr | [(v, "")] <- reads verStr -> Just v
+    _ -> Nothing
+  _ -> Nothing
+
 -- | Does the compiler support the -reexported-modules "A as B" syntax
 reexportedAsSupported :: Compiler -> Bool
 reexportedAsSupported comp = case compilerFlavor comp of

cabal-install/cabal-install.cabal

@@ -270,7 +270,7 @@ library
       , regex-base  >= 0.94.0.0 && <0.95
       , regex-posix >= 0.96.0.0 && <0.97
       , safe-exceptions >= 0.1.7.0 && < 0.2
-      , semaphore-compat >= 1.0.0 && < 1.1
+      , semaphore-compat >= 2.0.0 && < 2.1
 
     if flag(native-dns)
       if os(windows)

cabal-install/src/Distribution/Client/JobControl.hs

@@ -183,26 +183,34 @@ newSemaphoreJobControl _ n
   | n < 1 || n > 1000 =
       error $ "newParallelJobControl: not a sensible number of jobs: " ++ show n
 newSemaphoreJobControl verbosity maxJobLimit = do
-  sem <- freshSemaphore "cabal_semaphore" maxJobLimit
-  info verbosity $
-    "Created semaphore called "
-      ++ getSemaphoreName (semaphoreName sem)
-      ++ " with "
-      ++ show maxJobLimit
-      ++ " slots."
-  outqVar <- newTChanIO
-  inqVar <- newTChanIO
-  countVar <- newTVarIO 0
-  void (forkIO (worker sem inqVar outqVar))
-  return
-    JobControl
-      { spawnJob = spawn inqVar countVar
-      , collectJob = collect outqVar countVar
-      , remainingJobs = remaining countVar
-      , cancelJobs = cancel inqVar countVar
-      , cleanupJobControl = destroySemaphore sem
-      , jobControlSemaphore = Just (semaphoreName sem)
-      }
+  mbServer <- freshSemaphore "cabal_semaphore" maxJobLimit
+  case mbServer of
+    Left err -> do
+      warn verbosity $
+        "Failed to create semaphore: " ++ show err
+          ++ "; falling back to normal parallelism control."
+      newParallelJobControl maxJobLimit
+    Right server -> do
+      let sem = serverSemaphore server
+      info verbosity $
+        "Created semaphore called "
+          ++ getSemaphoreName (semaphoreName sem)
+          ++ " with "
+          ++ show maxJobLimit
+          ++ " slots."
+      outqVar <- newTChanIO
+      inqVar <- newTChanIO
+      countVar <- newTVarIO 0
+      void (forkIO (worker sem inqVar outqVar))
+      return
+        JobControl
+          { spawnJob = spawn inqVar countVar
+          , collectJob = collect outqVar countVar
+          , remainingJobs = remaining countVar
+          , cancelJobs = cancel inqVar countVar
+          , cleanupJobControl = destroySemaphoreServer server
+          , jobControlSemaphore = Just (semaphoreName sem)
+          }
   where
     worker :: Semaphore -> TChan (IO a) -> TChan (Either SomeException a) -> IO ()
     worker sem inqVar outqVar =
@@ -291,8 +299,18 @@ newJobControlFromParStrat verbosity mcompiler parStrat numJobsCap = case parStra
   UseSem n ->
     case mcompiler of
       Just compiler
-        | jsemSupported compiler ->
+        | jsemSupported compiler
+        , isJsemCompatible compiler ->
             newSemaphoreJobControl verbosity (capJobs n)
+        | jsemSupported compiler ->
+            do
+              warn verbosity $
+                "Semaphore version mismatch (cabal-install uses v"
+                ++ show semaphoreVersion
+                ++ ", but the selected GHC reports "
+                ++ maybe "no version (assumed v1)" (\v -> "v" ++ show v) (jsemVersion compiler)
+                ++ "); not using -jsem, GHC will be invoked without semaphore-based parallelism."
+              newParallelJobControl (capJobs n)
         | otherwise ->
             do
               warn verbosity "-jsem is not supported by the selected compiler, falling back to normal parallelism control."
@@ -303,6 +321,17 @@ newJobControlFromParStrat verbosity mcompiler parStrat numJobsCap = case parStra
   where
     capJobs n = min (fromMaybe maxBound numJobsCap) n
 
+-- | Check if the compiler's semaphore version is compatible with ours.
+--
+-- If the compiler doesn't report a "Semaphore version" field (GHC 9.8–9.14),
+-- we assume v1. On POSIX, v1 and v2 are incompatible (different mechanisms).
+-- On Windows, all versions are compatible (same Win32 API).
+isJsemCompatible :: Compiler -> Bool
+isJsemCompatible compiler =
+  case jsemVersion compiler of
+    Just v  -> versionsAreCompatible v semaphoreVersion
+    Nothing -> versionsAreCompatible 1 semaphoreVersion
+
 withJobControl :: IO (JobControl IO a) -> (JobControl IO a -> IO b) -> IO b
 withJobControl mkJC = bracket mkJC cleanupJobControl
 

changelog.d/semaphore-version-compat.md

@@ -0,0 +1,21 @@
+---
+synopsis: Detect semaphore version mismatch between cabal-install and GHC
+packages: [Cabal, cabal-install]
+prs: 0000
+issues: 0000
+significance: significant
+---
+
+When using `--semaphore`, cabal-install now checks whether the selected GHC's
+semaphore protocol version is compatible before passing `-jsem`.  If the GHC
+reports no `Semaphore version` field (GHC 9.8–9.14, which use v1) and
+cabal-install uses v2, a warning is emitted and cabal-install falls back to
+normal parallelism control instead of passing an incompatible semaphore name.
+
+On Windows, v1 and v2 are always compatible (same Win32 API), so semaphore
+coordination is preserved across all version combinations.
+
+- `Cabal`: add `jsemVersion :: Compiler -> Maybe Int` to read the
+  `Semaphore version` field from `ghc --info`.
+- `cabal-install`: add `isJsemCompatible` check in `newJobControlFromParStrat`;
+  emit a warning and fall back to `-jN` when versions are incompatible.