Use base_path as a relative folder from conanfile.source_folder

conan/tools/files/patches.py

@@ -25,9 +25,10 @@ def emit(self, record):
             self._output.info("%s: %s" % (self.patchname, logstr))
 
 
-def patch(conanfile, patch_file=None, patch_string=None, strip=0, fuzz=False, **kwargs):
+def patch(conanfile, base_path=None, patch_file=None, patch_string=None, strip=0, fuzz=False, **kwargs):
     """ Applies a diff from file (patch_file)  or string (patch_string)
         in base_path directory or current dir if None
+    :param base_path: Base path where the patch should be applied.
     :param patch_file: Patch file that should be applied.
     :param patch_string: Patch string that should be applied.
     :param strip: Number of folders to be stripped from the path.
@@ -56,7 +57,8 @@ def patch(conanfile, patch_file=None, patch_string=None, strip=0, fuzz=False, **
     if not patchset:
         raise ConanException("Failed to parse patch: %s" % (patch_file if patch_file else "string"))
 
-    if not patchset.apply(root=conanfile.source_folder, strip=strip, fuzz=fuzz):
+    root = os.path.join(conanfile.source_folder, base_path) if base_path else conanfile.source_folder
+    if not patchset.apply(root, strip=strip, fuzz=fuzz):
         raise ConanException("Failed to apply patch: %s" % patch_file)
 
 
@@ -71,6 +73,7 @@ def apply_conandata_patches(conanfile):
     ```
     patches:
     - patch_file: "patches/0001-buildflatbuffers-cmake.patch"
+      base_path: "source_subfolder"
     - patch_file: "patches/0002-implicit-copy-constructor.patch"
       patch_type: backport
       patch_source: https://github.com/google/flatbuffers/pull/5650

conans/test/functional/tools/test_files.py

@@ -158,6 +158,36 @@ def build(self):
            ' clang compilers.' in str(client.out)
 
 
+def test_apply_conandata_patches_relative_base_path(mock_patch_ng):
+    conanfile = textwrap.dedent("""
+        from conans import ConanFile
+        from conan.tools.files import apply_conandata_patches
+
+        class Pkg(ConanFile):
+            name = "mypkg"
+            version = "1.11.0"
+
+            def layout(self):
+                self.folders.source = "source_subfolder"
+
+            def build(self):
+                apply_conandata_patches(self)
+        """)
+    conandata_yml = textwrap.dedent("""
+        patches:
+          "1.11.0":
+            - patch_file: "patches/0001-buildflatbuffers-cmake.patch"
+              base_path: "relative_dir"
+    """)
+
+    client = TestClient()
+    client.save({'conanfile.py': conanfile,
+                 'conandata.yml': conandata_yml})
+    client.run('create .')
+
+    assert mock_patch_ng.apply_args[0].endswith(os.path.join('source_subfolder', "relative_dir"))
+    assert mock_patch_ng.apply_args[1:] == (0, False)
+
 
 def test_no_patch_file_entry():
     conanfile = textwrap.dedent("""

conans/test/unittests/tools/files/test_patches.py

@@ -1,3 +1,5 @@
+import os
+
 import patch_ng
 import pytest
 
@@ -44,6 +46,17 @@ def test_single_patch_file(mock_patch_ng):
     assert len(str(conanfile.output)) == 0
 
 
+def test_base_path(mock_patch_ng):
+    conanfile = ConanFileMock()
+    conanfile.folders.set_base_source("my_source")
+    conanfile.display_name = 'mocked/ref'
+    patch(conanfile, patch_file='patch-file', base_path="subfolder")
+    assert mock_patch_ng.filename == 'patch-file'
+    assert mock_patch_ng.string is None
+    assert mock_patch_ng.apply_args == (os.path.join("my_source", "subfolder"), 0, False)
+    assert len(str(conanfile.output)) == 0
+
+
 def test_single_patch_string(mock_patch_ng):
     conanfile = ConanFileMock()
     conanfile.folders.set_base_source("my_folder")