Unix: improve SocketError.SocketError Exception Message (dotnet#38073)

* Unix: improve SocketError.SocketError Exception Message

This introduces a fake errno for SocketError.SocketError, which
maps to an error message of "Unknown socket error".

* PR feedback

Co-authored-by: Stephen Toub <stoub@microsoft.com>

Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/libraries/Common/src/Interop/Unix/Interop.Errors.cs

@@ -103,6 +103,7 @@ internal enum Error
 
         // Custom Error codes to track errors beyond kernel interface.
         EHOSTNOTFOUND    = 0x20001,           // Name lookup failed
+        ESOCKETERROR     = 0x20002,           // Unspecified socket error
 
         // POSIX permits these to have the same value and we make them always equal so
         // that we do not introduce a dependency on distinguishing between them that

src/libraries/Common/src/System/Net/Sockets/SocketErrorPal.Unix.cs

@@ -21,7 +21,7 @@ static SocketErrorPal()
 #endif
 
         private const int NativeErrorToSocketErrorCount = 42;
-        private const int SocketErrorToNativeErrorCount = 40;
+        private const int SocketErrorToNativeErrorCount = 41;
 
         // No Interop.Errors are included for the following SocketErrors, as there's no good mapping:
         // - SocketError.NoRecovery
@@ -126,6 +126,7 @@ static SocketErrorPal()
             { SocketError.TooManyOpenSockets, Interop.Error.ENFILE }, // could also have been EMFILE
             { SocketError.TryAgain, Interop.Error.EAGAIN }, // not a perfect mapping, but better than nothing
             { SocketError.WouldBlock, Interop.Error.EAGAIN  },
+            { SocketError.SocketError, Interop.Error.ESOCKETERROR },
         };
 
         internal static SocketError GetSocketErrorForNativeError(Interop.Error errno)

src/libraries/Native/Unix/Common/pal_error_common.h

@@ -124,6 +124,7 @@ typedef enum
 
     // Error codes to track errors beyond kernel.
     Error_EHOSTNOTFOUND = 0x20001,   // Name lookup failed.
+    Error_ESOCKETERROR = 0x20002,    // Unidentified socket error.
 
     // POSIX permits these to have the same value and we make them
     // always equal so that we cannot introduce a dependency on
@@ -137,6 +138,17 @@ typedef enum
     Error_ENONSTANDARD = 0x1FFFF,
 } Error;
 
+/*
+  Some pal errors don't have a corresponding errno value.
+  We define values for these errors.
+  We want these values to be distinct from real errno values.
+  We base of the Error enum values which are chosen to be out of the
+  typical errno range, and make them negative because POSIX
+  requires errno values to be positive.
+*/
+#define EHOSTNOTFOUND (-Error_EHOSTNOTFOUND)
+#define ESOCKETERROR  (-Error_ESOCKETERROR)
+
 inline static int32_t ConvertErrorPlatformToPal(int32_t platformErrno)
 {
     switch (platformErrno)
@@ -500,7 +512,10 @@ inline static int32_t ConvertErrorPalToPlatform(int32_t error)
         case Error_ENODATA:
             return ENODATA;
         case Error_EHOSTNOTFOUND:
-            return -(Error_EHOSTNOTFOUND);
+            return EHOSTNOTFOUND;
+        case Error_ESOCKETERROR:
+            return ESOCKETERROR;
+
         case Error_ENONSTANDARD:
             break; // fall through to assert
     }
@@ -518,16 +533,19 @@ inline static int32_t ConvertErrorPalToPlatform(int32_t error)
     return -1;
 }
 
-static int32_t ConvertErrorPalToGai(int32_t error)
+static bool TryConvertErrorToGai(int32_t error, int32_t* gaiError)
 {
+    assert(gaiError != NULL);
+
     switch (error)
     {
-        case -(Error_EHOSTNOTFOUND):
-            return EAI_NONAME;
+        case EHOSTNOTFOUND:
+            *gaiError = EAI_NONAME;
+            return true;
+        default:
+            *gaiError = error;
+            return false;
     }
-    // Fall-through for unknown codes. gai_strerror() will handle that.
-
-    return error;
 }
 
 
@@ -543,8 +561,17 @@ inline static const char* StrErrorR(int32_t platformErrno, char* buffer, int32_t
     if (platformErrno < 0)
     {
         // Not a system error
-        SafeStringCopy(buffer, (size_t)bufferSize, gai_strerror(ConvertErrorPalToGai(platformErrno)));
-        return buffer;
+        int32_t gaiError;
+        if (TryConvertErrorToGai(platformErrno, &gaiError))
+        {
+            SafeStringCopy(buffer, (size_t)bufferSize, gai_strerror(gaiError));
+            return buffer;
+        }
+        else if (platformErrno == ESOCKETERROR)
+        {
+            SafeStringCopy(buffer, (size_t)bufferSize, "Unknown socket error");
+            return buffer;
+        }
     }
 
 // Note that we must use strerror_r because plain strerror is not

src/libraries/System.Net.Primitives/src/System/Net/SocketException.Unix.cs

@@ -26,15 +26,11 @@ private SocketException(Interop.ErrorInfo error) : this(SocketErrorPal.GetSocket
         private static int GetNativeErrorForSocketError(SocketError error)
         {
             int nativeErr = (int)error;
-            if (error != SocketError.SocketError)
-            {
-                Interop.Error interopErr;
 
-                // If an interop error was not found, then don't invoke Info().RawErrno as that will fail with assert.
-                if (SocketErrorPal.TryGetNativeErrorForSocketError(error, out interopErr))
-                {
-                    nativeErr = interopErr.Info().RawErrno;
-                }
+            // If an interop error was not found, then don't invoke Info().RawErrno as that will fail with assert.
+            if (SocketErrorPal.TryGetNativeErrorForSocketError(error, out Interop.Error interopErr))
+            {
+                nativeErr = interopErr.Info().RawErrno;
             }
 
             return nativeErr;