Use absolute paths for external symlinks in Builder#copyDirectory.

Thanks to @worldsayshi for reporting this issue with a reproduction.

Fixes meteor#8005.
Fixes meteor#2876.
Fixes meteor#7154.

tools/isobuild/builder.js

@@ -1,3 +1,4 @@
+import assert from "assert";
 import {WatchSet, readAndWatchFile, sha1} from '../fs/watch.js';
 import files from '../fs/files.js';
 import NpmDiscards from './npm-discards.js';
@@ -65,6 +66,8 @@ export default class Builder {
     this.writtenHashes = {};
     this.previousWrittenHashes = {};
 
+    this._realpathCache = Object.create(null);
+
     // foo/bar => foo/.build1234.bar
     // Should we include a random number? The advantage is that multiple
     // builds can run in parallel. The disadvantage is that stale build
@@ -443,7 +446,11 @@ Previous builder: ${previousBuilder.outputPath}, this builder: ${outputPath}`
       });
     }
 
-    let walk = (absFrom, relTo) => {
+    const walk = (
+      absFrom,
+      relTo,
+      _currentRealRootDir = absFrom
+    ) => {
       if (symlink && ! (relTo in this.usedAsFile)) {
         this._ensureDirectory(files.pathDirname(relTo));
         const absTo = files.pathResolve(this.buildPath, relTo);
@@ -454,14 +461,55 @@ Previous builder: ${previousBuilder.outputPath}, this builder: ${outputPath}`
       this._ensureDirectory(relTo);
 
       optimisticReaddir(absFrom).forEach(item => {
-        const thisAbsFrom = files.pathResolve(absFrom, item);
+        let thisAbsFrom = files.pathResolve(absFrom, item);
         const thisRelTo = files.pathJoin(relTo, item);
 
         if (specificPaths && !(thisRelTo in specificPaths)) {
           return;
         }
 
-        const fileStatus = optimisticLStat(thisAbsFrom);
+        // Returns files.realpath(thisAbsFrom), iff it is external to
+        // _currentRealRootDir, using caching because this function might
+        // be called more than once.
+        let cachedExternalPath;
+        const getExternalPath = () => {
+          if (typeof cachedExternalPath !== "undefined") {
+            return cachedExternalPath;
+          }
+
+          const real = files.realpath(thisAbsFrom, this._realpathCache);
+          const isExternal =
+            files.pathRelative(_currentRealRootDir, real).startsWith("..");
+
+          // Now cachedExternalPath is either a string or false.
+          return cachedExternalPath = isExternal && real;
+        };
+
+        let fileStatus = optimisticLStat(thisAbsFrom);
+
+        if (! symlink && fileStatus.isSymbolicLink()) {
+          // If copyDirectory is not allowed to create symbolic links to
+          // external files, and this file is a symbolic link that points
+          // to an external file, update fileStatus so that we copy this
+          // file as a normal file rather than as a symbolic link.
+
+          const externalPath = getExternalPath();
+          if (externalPath) {
+            // Copy from the real path rather than the link path.
+            thisAbsFrom = externalPath;
+
+            // Update fileStatus to match the actual file rather than the
+            // symbolic link, thus forcing the file to be copied below.
+            fileStatus = optimisticLStat(externalPath);
+
+            if (fileStatus.isDirectory()) {
+              // Update _currentRealRootDir so that we can judge
+              // isExternal relative to this new root directory when
+              // traversing nested directories.
+              _currentRealRootDir = externalPath;
+            }
+          }
+        }
 
         let itemForMatch = item;
         const isDirectory = fileStatus.isDirectory();
@@ -482,12 +530,15 @@ Previous builder: ${previousBuilder.outputPath}, this builder: ${outputPath}`
         if (isDirectory) {
           if (typeof directoryFilter !== "function" ||
               directoryFilter(thisAbsFrom)) {
-            walk(thisAbsFrom, thisRelTo);
+            walk(thisAbsFrom, thisRelTo, _currentRealRootDir);
           }
 
         } else if (fileStatus.isSymbolicLink()) {
           symlinkWithOverwrite(
-            files.readlink(thisAbsFrom),
+            // Symbolic links pointing to relative external paths are less
+            // portable than absolute links, so getExternalPath() is
+            // preferred if it returns a path.
+            getExternalPath() || files.readlink(thisAbsFrom),
             files.pathResolve(this.buildPath, thisRelTo)
           );
 
@@ -518,7 +569,7 @@ Previous builder: ${previousBuilder.outputPath}, this builder: ${outputPath}`
       });
     };
 
-    walk(from, to);
+    walk(files.realpath(from, this._realpathCache), to);
   }
 
   // Returns a new Builder-compatible object that works just like a