Avoid PATH_MAX when getting the executable path on Linux and OpenBSD. (#1403)

This PR adjusts the implementations of `CommandLine.executablePath` on
Linux and OpenBSD to avoid an artificial upper bound based on
`PATH_MAX`.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/Support/Additions/CommandLineAdditions.swift

@@ -33,14 +33,27 @@ extension CommandLine {
       }
       return result!
 #elseif os(Linux) || os(Android)
-      return try withUnsafeTemporaryAllocation(of: CChar.self, capacity: Int(PATH_MAX) * 2) { buffer in
-        let readCount = readlink("/proc/self/exe", buffer.baseAddress!, buffer.count - 1)
-        guard readCount >= 0 else {
-          throw CError(rawValue: swt_errno())
+      var result: String?
+#if DEBUG
+      var bufferCount = Int(1) // force looping
+#else
+      var bufferCount = Int(PATH_MAX)
+#endif
+      while result == nil {
+        try withUnsafeTemporaryAllocation(of: CChar.self, capacity: bufferCount) { buffer in
+          let readCount = readlink("/proc/self/exe", buffer.baseAddress!, buffer.count)
+          guard readCount >= 0 else {
+            throw CError(rawValue: swt_errno())
+          }
+          if readCount < buffer.count {
+            buffer[readCount] = 0 // NUL-terminate the string.
+            result = String(cString: buffer.baseAddress!)
+          } else {
+            bufferCount += Int(PATH_MAX) // add more space and try again
+          }
         }
-        buffer[readCount] = 0 // NUL-terminate the string.
-        return String(cString: buffer.baseAddress!)
       }
+      return result!
 #elseif os(FreeBSD)
       var mib = [CTL_KERN, KERN_PROC, KERN_PROC_PATHNAME, -1]
       return try mib.withUnsafeMutableBufferPointer { mib in

Sources/_TestingInternals/ExecutablePath.cpp

@@ -24,9 +24,8 @@ static constinit std::atomic<const char *> earlyCWD { nullptr };
 /// path (which can occur when manually invoking the test executable.)
 __attribute__((__constructor__(101), __used__))
 static void captureEarlyCWD(void) {
-  static char buffer[PATH_MAX * 2];
-  if (getcwd(buffer, sizeof(buffer))) {
-    earlyCWD.store(buffer);
+  if (auto cwd = getcwd(nil, 0)) {
+    earlyCWD.store(cwd);
   }
 }
 #endif