Auto-detect free-threaded libpython in venv home

greateggsgreg · filmor · commit a30c84ec7fa7 · 2026-05-15T22:21:02.000+02:00
PythonEnvironment.FindLibPythonInHome built a single candidate name from version.Major.Minor (e.g. libpython3.14.so) and missed the free-threaded variant (libpython3.14t.so / python314t.dll). pyvenv.cfg's version field doesn't distinguish the two builds, so probe both names and let File.Exists pick the one that's actually on disk. Unblocks the 3.14t CI jobs added in #2721: they were failing in PythonEngine.Initialize with "Py_IncRef: undefined symbol" because PythonDLL resolved to null and pythonnet fell back to dlopen of the dotnet binary itself.
diff --git a/src/runtime/Util/PythonEnvironment.cs b/src/runtime/Util/PythonEnvironment.cs
@@ -132,7 +132,12 @@ private static Dictionary<string, string> TryParse(string venvCfg)
 
     private static string? FindLibPythonInHome(string home, Version version)
     {
-        var libPythonName = GetDefaultDllName(version);
+        // Probe both — pyvenv.cfg's version field doesn't distinguish free-threaded.
+        var libPythonNames = new[]
+        {
+            GetDefaultDllName(version),
+            GetDefaultDllName(version, freeThreaded: true),
+        };
 
         List<string> pathsToCheck = new();
         if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux))
@@ -155,7 +160,7 @@ private static Dictionary<string, string> TryParse(string venvCfg)
         }
 
         return pathsToCheck
-            .Select(path => Path.Combine(home, path, libPythonName))
+            .SelectMany(path => libPythonNames.Select(name => Path.Combine(home, path, name)))
             .FirstOrDefault(File.Exists);
     }
 
@@ -171,13 +176,15 @@ private static string ProgramNameFromPath(string path)
         }
     }
 
-    internal static string GetDefaultDllName(Version version)
+    internal static string GetDefaultDllName(Version version, bool freeThreaded = false)
     {
         string prefix = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? "" : "lib";
 
+        string ftSuffix = freeThreaded ? "t" : "";
+
         string suffix = RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
-            ? Invariant($"{version.Major}{version.Minor}")
-            : Invariant($"{version.Major}.{version.Minor}");
+            ? Invariant($"{version.Major}{version.Minor}{ftSuffix}")
+            : Invariant($"{version.Major}.{version.Minor}{ftSuffix}");
 
         string ext = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? ".dll"
             : RuntimeInformation.IsOSPlatform(OSPlatform.OSX) ? ".dylib"

Original file line number	Diff line number	Diff line change
`@@ -132,7 +132,12 @@ private static Dictionary<string, string> TryParse(string venvCfg)`
`132`	`132`
`133`	`133`	`private static string? FindLibPythonInHome(string home, Version version)`
`134`	`134`	`{`
`135`		`- var libPythonName = GetDefaultDllName(version);`
	`135`	`+ // Probe both — pyvenv.cfg's version field doesn't distinguish free-threaded.`
	`136`	`+ var libPythonNames = new[]`
	`137`	`+ {`
	`138`	`+ GetDefaultDllName(version),`
	`139`	`+ GetDefaultDllName(version, freeThreaded: true),`
	`140`	`+ };`
`136`	`141`
`137`	`142`	`List<string> pathsToCheck = new();`
`138`	`143`	`if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux))`
`@@ -155,7 +160,7 @@ private static Dictionary<string, string> TryParse(string venvCfg)`
`155`	`160`	`}`
`156`	`161`
`157`	`162`	`return pathsToCheck`
`158`		`- .Select(path => Path.Combine(home, path, libPythonName))`
	`163`	`+ .SelectMany(path => libPythonNames.Select(name => Path.Combine(home, path, name)))`
`159`	`164`	`.FirstOrDefault(File.Exists);`
`160`	`165`	`}`
`161`	`166`
`@@ -171,13 +176,15 @@ private static string ProgramNameFromPath(string path)`
`171`	`176`	`}`
`172`	`177`	`}`
`173`	`178`
`174`		`- internal static string GetDefaultDllName(Version version)`
	`179`	`+ internal static string GetDefaultDllName(Version version, bool freeThreaded = false)`
`175`	`180`	`{`
`176`	`181`	`string prefix = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? "" : "lib";`
`177`	`182`
	`183`	`+ string ftSuffix = freeThreaded ? "t" : "";`
	`184`	`+`
`178`	`185`	`string suffix = RuntimeInformation.IsOSPlatform(OSPlatform.Windows)`
`179`		`- ? Invariant($"{version.Major}{version.Minor}")`
`180`		`- : Invariant($"{version.Major}.{version.Minor}");`
	`186`	`+ ? Invariant($"{version.Major}{version.Minor}{ftSuffix}")`
	`187`	`+ : Invariant($"{version.Major}.{version.Minor}{ftSuffix}");`
`181`	`188`
`182`	`189`	`string ext = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? ".dll"`
`183`	`190`	`: RuntimeInformation.IsOSPlatform(OSPlatform.OSX) ? ".dylib"`