Prevent --multiple-file-output-path writes from following symlinks outside the target directory (#1467)

pkl-cli/src/main/kotlin/org/pkl/cli/CliEvaluator.kt

@@ -240,8 +240,7 @@ constructor(
 
       for ((pathSpec, fileOutput) in output) {
         checkPathSpec(pathSpec)
-        val resolvedPath = realOutputDir.resolve(pathSpec).normalize()
+        val (realPath, resolvedPath) = realOutputDir.resolveRealPath(Path.of(pathSpec))
-        val realPath = if (resolvedPath.exists()) resolvedPath.toRealPath() else resolvedPath
         if (!realPath.startsWith(realOutputDir)) {
           throw CliException(
             "Output file conflict: `output.files` entry `\"$pathSpec\"` in module `$moduleUri` resolves to file path `$realPath`, which is outside output directory `$realOutputDir`."
@@ -269,4 +268,22 @@ constructor(
       }
     }
   }
+
+  /**
+   * Resolves [rel] against this Path name-by-name. At each step, the real path is resolved if the
+   * file exists. The normalized real path and normalized resolved path are returned. This has a
+   * similar effect to `this.resolve(rel).toRealPath().normalize()`, but the real paths account for
+   * symlinks in the middle of the relative path so the full path need not exist.
+   */
+  private fun Path.resolveRealPath(rel: Path): Pair<Path, Path> {
+    assert(!rel.isAbsolute)
+    var resolved = this
+    var real = this
+    for (name in rel) {
+      resolved = resolved.resolve(name)
+      real = real.resolve(name)
+      if (real.exists()) real = real.toRealPath()
+    }
+    return real.normalize() to resolved.normalize()
+  }
 }

pkl-cli/src/test/kotlin/org/pkl/cli/CliEvaluatorTest.kt

@@ -931,6 +931,37 @@ result = someLib.x
       .hasMessageContaining("which is outside output directory")
   }
 
+  @Test
+  @DisabledOnOs(OS.WINDOWS)
+  fun `multiple file output throws if files are written outside the base path via symlink`() {
+    val output = tempDir.resolve(".output").createDirectories()
+    val outside = tempDir.resolve("outside").createDirectories()
+    output.resolve("outside").createSymbolicLinkPointingTo(outside)
+
+    val moduleUri =
+      writePklFile(
+        "test.pkl",
+        """
+        output {
+          files {
+            ["outside/foo.txt"] {
+              text = "bar"
+            }
+          }
+        }
+      """
+          .trimIndent(),
+      )
+    val options =
+      CliEvaluatorOptions(
+        CliBaseOptions(sourceModules = listOf(moduleUri), workingDir = tempDir),
+        multipleFileOutputPath = ".output",
+      )
+    assertThatCode { evalToConsole(options) }
+      .hasMessageStartingWith("Output file conflict:")
+      .hasMessageContaining("which is outside output directory")
+  }
+
   @Test
   fun `multiple file output throws if file path is a directory`() {
     tempDir.resolve(".output/myDir").createDirectories()