Merged PR 785522: [Linux sandbox] Don't resolve paths that we got from /proc/self/fd. Add test coverage for intermediate symlink resolution

The paths we get from `/proc/self/fd` are already resolved

Public/Src/Engine/UnitTests/Processes/LinuxSandboxProcessTests.cs

@@ -105,5 +105,42 @@ namespace Test.BuildXL.Processes
             AssertLogContains(GetRegex("readlink", "realDir/symlink.txt"));
             AssertLogNotContains(GetRegex("readlink", "symlinkDir/file.txt"));
         }
+
+        [FactIfSupported(requiresSymlinkPermission: true)]
+        public void FileDescriptorAccessesFullyResolvesPath()
+        {
+            var tempFiles = new TempFileStorage(canGetFileNames : true);
+            var link = tempFiles.GetDirectory("symlinkDir", skipCreate: true);
+            var real = tempFiles.GetDirectory("realDir");
+            var realFile = tempFiles.GetFileName(real, "file.txt");
+
+            File.WriteAllText(realFile, "chelivery");
+            XAssert.IsTrue(File.Exists(realFile));           
+
+            var createSymlink = FileUtilities.TryCreateSymbolicLink(link, real, isTargetFile: false); 
+            if (!createSymlink.Succeeded)
+            {
+                XAssert.IsTrue(false, createSymlink.Failure.Describe());                
+            }
+
+            var fileLink = tempFiles.GetFileName(real, "symlink.txt");
+            createSymlink = FileUtilities.TryCreateSymbolicLink(fileLink, realFile, isTargetFile: true); 
+            if (!createSymlink.Succeeded)
+            {
+                XAssert.IsTrue(false, createSymlink.Failure.Describe());                
+            }
+
+            RunNativeTest("FileDescriptorAccessesFullyResolvesPath", workingDirectory: tempFiles);
+
+            // The native side does:
+            // fd = open(symlinkDir/symlink.txt);
+            // __fxstat(fd)
+            // For the __fxstat report, we should associate the file descriptor to the real path, with the symlinks resolved
+            AssertLogContains(GetRegex("__fxstat", realFile));
+
+            // At some point (namely, on open) we also should get reports for the intermediate symlinks that got us to the file 
+            AssertLogContains(GetRegex("_readlink", link));
+            AssertLogContains(GetRegex("_readlink", fileLink));
+        }
     }
 }

Public/Src/Sandbox/Linux/UnitTests/TestProcesses/TestProcess/main.cpp

@@ -54,6 +54,23 @@ int ReadlinkReportDoesNotResolveFinalComponent()
     return EXIT_SUCCESS;
 }
 
+// The managed side creates:
+// - a directory symlink realDir -> symlinkDir
+// - a file symlink realDir/symlink.txt -> realDir/real.txt.
+int FileDescriptorAccessesFullyResolvesPath()
+{
+    char buf[PATH_MAX] = { 0 };
+    GET_CWD;
+    std::string testFile(cwd);
+    testFile.append("/realDir/symlink.txt");
+    int fd = open("symlinkDir/symlink.txt", O_RDONLY);
+    struct stat sb;
+    
+    // Use __fxtat as representative for a "file descriptor event"
+    __fxstat(1, fd, &sb);
+    return EXIT_SUCCESS;
+}
+
 
 int main(int argc, char **argv)
 {
@@ -191,6 +208,7 @@ int main(int argc, char **argv)
     IF_COMMAND(TestAnonymousFile);
     IF_COMMAND(FullPathResolutionOnReports);
     IF_COMMAND(ReadlinkReportDoesNotResolveFinalComponent);
+    IF_COMMAND(FileDescriptorAccessesFullyResolvesPath);
 
     // Invalid command
     exit(-1);

Public/Src/Sandbox/Linux/bxl_observer.cpp

@@ -290,7 +290,10 @@ bool BxlObserver::ResolveEventPaths(buildxl::linux::SandboxEvent& event) {
             if (event.GetDstFd() != -1) {
                 FileDescriptorToPath(event.GetDstFd(), event.GetPid(), resolved_path_dst, PATH_MAX);
             }
-            ResolveEventPaths(event, resolved_path_src, resolved_path_dst);
+
+            // FileDescriptorToPath returns a fully resolved path (by virtue of resolving /proc/self/fd/{fd})
+            // so no need to call ResolveEventPaths here, we can just set them directly
+            event.SetResolvedPaths(resolved_path_src, resolved_path_dst);
             break;
         }
         case buildxl::linux::SandboxEventPathType::kRelativePaths: {