[VFS] Add a "redirecting-with" field to overlays

clang/test/VFS/Inputs/redirect-and-fallthrough.yaml

@@ -0,0 +1,11 @@
+{
+  'redirecting-with': 'fallthrough',
+  'fallthrough': true,
+  'roots': [
+    {
+      'type': 'directory-remap',
+      'name': '//root/a',
+      'external-contents': '//root/b'
+    }
+  ]
+}

clang/test/VFS/Inputs/unknown-redirect.yaml

@@ -0,0 +1,10 @@
+{
+  'redirecting-with': 'none',
+  'roots': [
+    {
+      'type': 'directory-remap',
+      'name': '//root/a',
+      'external-contents': '//root/b'
+    }
+  ]
+}

clang/test/VFS/fallback.c

@@ -0,0 +1,86 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+// Test fallback directory remapping, ie. a directory "Base" which is used as
+// a fallback if files are missing from "UseFirst"
+
+// RUN: sed -e "s@EXTERNAL_DIR@%{/t:regex_replacement}/Both/Base@g" -e "s@NAME_DIR@%{/t:regex_replacement}/Both/UseFirst@g" %t/vfs/base.yaml > %t/vfs/both.yaml
+
+// RUN: cp -R %t/Both %t/UseFirstOnly
+// RUN: rm -rf %t/UseFirstOnly/Base
+// RUN: sed -e "s@EXTERNAL_DIR@%{/t:regex_replacement}/UseFirstOnly/Base@g" -e "s@NAME_DIR@%{/t:regex_replacement}/UseFirstOnly/UseFirst@g" %t/vfs/base.yaml > %t/vfs/use-first-only.yaml
+
+// RUN: cp -R %t/Both %t/BaseOnly
+// RUN: rm -rf %t/BaseOnly/UseFirst
+// RUN: sed -e "s@EXTERNAL_DIR@%{/t:regex_replacement}/BaseOnly/Base@g" -e "s@NAME_DIR@%{/t:regex_replacement}/BaseOnly/UseFirst@g" %t/vfs/base.yaml > %t/vfs/base-only.yaml
+
+// RUN: cp -R %t/Both %t/BFallback
+// RUN: rm %t/BFallback/UseFirst/B.h
+// RUN: sed -e "s@EXTERNAL_DIR@%{/t:regex_replacement}/BFallback/Base@g" -e "s@NAME_DIR@%{/t:regex_replacement}/BFallback/UseFirst@g" %t/vfs/base.yaml > %t/vfs/b-fallback.yaml
+
+// RUN: cp -R %t/Both %t/CFallback
+// RUN: rm %t/CFallback/UseFirst/C.h
+// RUN: sed -e "s@EXTERNAL_DIR@%{/t:regex_replacement}/CFallback/Base@g" -e "s@NAME_DIR@%{/t:regex_replacement}/CFallback/UseFirst@g" %t/vfs/base.yaml > %t/vfs/c-fallback.yaml
+
+// Both B.h and C.h are in both folders
+// RUN: %clang_cc1 -Werror -I %t/Both/UseFirst -ivfsoverlay %t/vfs/both.yaml -fsyntax-only -E -C %t/main.c 2>&1 | FileCheck --check-prefix=IN_UF %s
+
+// IN_UF: # 1 "{{.*(/|\\\\)UseFirst(/|\\\\)}}B.h"
+// IN_UF-NEXT: // B.h in UseFirst
+// IN_UF: # 1 "{{.*(/|\\\\)UseFirst(/|\\\\)}}C.h"
+// IN_UF-NEXT: // C.h in UseFirst
+
+// Base missing, so now they are only in UseFirst
+// RUN: %clang_cc1 -Werror -I %t/UseFirstOnly/UseFirst -ivfsoverlay %t/vfs/use-first-only.yaml -fsyntax-only -E -C %t/main.c 2>&1 | FileCheck --check-prefix=IN_UF %s
+
+// UseFirst missing, fallback to Base
+// RUN: %clang_cc1 -Werror -I %t/BaseOnly/UseFirst -ivfsoverlay %t/vfs/base-only.yaml -fsyntax-only -E -C %t/main.c 2>&1 | FileCheck --check-prefix=IN_BASE %s
+
+// IN_BASE: # 1 "{{.*(/|\\\\)Base(/|\\\\)}}B.h"
+// IN_BASE-NEXT: // B.h in Base
+// IN_BASE: # 1 "{{.*(/|\\\\)Base(/|\\\\)}}C.h"
+// IN_BASE-NEXT: // C.h in Base
+
+// B.h missing from UseFirst
+// RUN: %clang_cc1 -Werror -I %t/BFallback/UseFirst -ivfsoverlay %t/vfs/b-fallback.yaml -fsyntax-only -E -C %t/main.c 2>&1 | FileCheck --check-prefix=B_FALLBACK %s
+
+// B_FALLBACK: # 1 "{{.*(/|\\\\)Base(/|\\\\)}}B.h"
+// B_FALLBACK-NEXT: // B.h in Base
+// B_FALLBACK: # 1 "{{.*(/|\\\\)UseFirst(/|\\\\)}}C.h"
+// B_FALLBACK-NEXT: // C.h in UseFirst
+
+// C.h missing from UseFirst
+// RUN: %clang_cc1 -Werror -I %t/CFallback/UseFirst -ivfsoverlay %t/vfs/c-fallback.yaml -fsyntax-only -E -C %t/main.c 2>&1 | FileCheck --check-prefix=C_FALLBACK %s
+
+// C_FALLBACK: # 1 "{{.*(/|\\\\)UseFirst(/|\\\\)}}B.h"
+// C_FALLBACK-NEXT: // B.h in UseFirst
+// C_FALLBACK: # 1 "{{.*(/|\\\\)Base(/|\\\\)}}C.h"
+// C_FALLBACK-NEXT: // C.h in Base
+
+//--- main.c
+#include "B.h"
+
+//--- Both/UseFirst/B.h
+// B.h in UseFirst
+#include "C.h"
+
+//--- Both/UseFirst/C.h
+// C.h in UseFirst
+
+//--- Both/Base/B.h
+// B.h in Base
+#include "C.h"
+
+//--- Both/Base/C.h
+// C.h in Base
+
+//--- vfs/base.yaml
+{
+  'version' : 0,
+      'redirecting-with' : 'fallback',
+                           'roots' : [
+                             {'name' : 'NAME_DIR',
+                              'type' : 'directory-remap',
+                              'external-contents' : 'EXTERNAL_DIR'}
+                           ]
+}

clang/test/VFS/parse-errors.c

@@ -12,3 +12,11 @@
 // RUN: not %clang_cc1 -ivfsoverlay %S/Inputs/unknown-value.yaml -fsyntax-only %s 2>&1 | FileCheck -check-prefix=CHECK-UNKNOWN-VALUE %s
 // CHECK-UNKNOWN-VALUE: expected boolean value
 // CHECK-UNKNOWN-VALUE: invalid virtual filesystem overlay file
+
+// RUN: not %clang_cc1 -ivfsoverlay %S/Inputs/unknown-redirect.yaml -fsyntax-only %s 2>&1 | FileCheck -check-prefix=CHECK-REDIRECT %s
+// CHECK-REDIRECT: expected valid redirect kind
+// CHECK-REDIRECT: invalid virtual filesystem overlay file
+
+// RUN: not %clang_cc1 -ivfsoverlay %S/Inputs/redirect-and-fallthrough.yaml -fsyntax-only %s 2>&1 | FileCheck -check-prefix=CHECK-EXCLUSIVE-KEYS %s
+// CHECK-EXCLUSIVE-KEYS: 'fallthrough' and 'redirecting-with' are mutually exclusive
+// CHECK-EXCLUSIVE-KEYS: invalid virtual filesystem overlay file

llvm/include/llvm/Support/VirtualFileSystem.h

@@ -549,7 +549,10 @@ class RedirectingFileSystemParser;
 ///   'case-sensitive': <boolean, default=(true for Posix, false for Windows)>
 ///   'use-external-names': <boolean, default=true>
 ///   'overlay-relative': <boolean, default=false>
-///   'fallthrough': <boolean, default=true>
+///   'fallthrough': <boolean, default=true, deprecated - use 'redirecting-with'
+///                   instead>
+///   'redirecting-with': <string, one of 'fallthrough', 'fallback', or
+///                        'redirect-only', default='fallthrough'>
 ///
 /// Virtual directories that list their contents are represented as
 /// \verbatim
@@ -620,6 +623,20 @@ class RedirectingFileSystem : public vfs::FileSystem {
   enum EntryKind { EK_Directory, EK_DirectoryRemap, EK_File };
   enum NameKind { NK_NotSet, NK_External, NK_Virtual };
 
+  /// The type of redirection to perform.
+  enum class RedirectKind {
+    /// Lookup the redirected path first (ie. the one specified in
+    /// 'external-contents') and if that fails "fallthrough" to a lookup of the
+    /// originally provided path.
+    Fallthrough,
+    /// Lookup the provided path first and if that fails, "fallback" to a
+    /// lookup of the redirected path.
+    Fallback,
+    /// Only lookup the redirected path, do not lookup the originally provided
+    /// path.
+    RedirectOnly
+  };
+
   /// A single file or directory in the VFS.
   class Entry {
     EntryKind Kind;
@@ -754,17 +771,11 @@ class RedirectingFileSystem : public vfs::FileSystem {
   friend class RedirectingFSDirIterImpl;
   friend class RedirectingFileSystemParser;
 
-  bool shouldUseExternalFS() const { return IsFallthrough; }
-
   /// Canonicalize path by removing ".", "..", "./", components. This is
   /// a VFS request, do not bother about symlinks in the path components
   /// but canonicalize in order to perform the correct entry search.
   std::error_code makeCanonical(SmallVectorImpl<char> &Path) const;
 
-  /// Whether to fall back to the external file system when an operation fails
-  /// with the given error code on a path associated with the provided Entry.
-  bool shouldFallBackToExternalFS(std::error_code EC, Entry *E = nullptr) const;
-
   /// Get the File status, or error, from the underlying external file system.
   /// This returns the status with the originally requested name, while looking
   /// up the entry using the canonical path.
@@ -817,9 +828,9 @@ class RedirectingFileSystem : public vfs::FileSystem {
   /// names of files.  This global value is overridable on a per-file basis.
   bool UseExternalNames = true;
 
-  /// Whether to attempt a file lookup in external file system after it wasn't
-  /// found in VFS.
-  bool IsFallthrough = true;
+  /// Determines the lookups to perform, as well as their order. See
+  /// \c RedirectKind for details.
+  RedirectKind Redirection = RedirectKind::Fallthrough;
   /// @}
 
   RedirectingFileSystem(IntrusiveRefCntPtr<FileSystem> ExternalFS);
@@ -874,8 +885,12 @@ class RedirectingFileSystem : public vfs::FileSystem {
 
   StringRef getExternalContentsPrefixDir() const;
 
+  /// Sets the redirection kind to \c Fallthrough if true or \c RedirectOnly
+  /// otherwise. Will removed in the future, use \c setRedirection instead.
   void setFallthrough(bool Fallthrough);
 
+  void setRedirection(RedirectingFileSystem::RedirectKind Kind);
+
   std::vector<llvm::StringRef> getRoots() const;
 
   void dump(raw_ostream &OS) const;