[skip ci] Update README.md, add instructions on how to make changes

README.md

@@ -1,20 +1,55 @@
 # yugabyte-db-thirdparty
 
-This repository contains Python-based automation to build and package third-party dependencies that are needed to build YugabyteDB. We package these dependencies as GitHub releases so that they can be downloaded by YugabyteDB CI/CD systems without having to rebuild them every time. Here is an example of how to build yugabyte-db-thirdparty and then YugabyteDB with GCC 9, although many GCC and Clang versionsare usable.
+This repository contains Python-based automation to build and package third-party dependencies that are needed to build YugabyteDB. We package these dependencies as GitHub releases so that they can be downloaded by YugabyteDB CI/CD systems without having to rebuild them every time. Here is an example of how to build yugabyte-db-thirdparty and then YugabyteDB with Clang 12.
 
 ```bash
 cd ~/code
 git clone https://github.com/yugabyte/yugabyte-db-thirdparty.git
 cd yugabyte-db-thirdparty
-./build_thirdparty.sh --devtoolset=9
+./build_thirdparty.sh --toolchain=llvm12
+```
+
+To use your local changes to yugabyte-db-thirdparty in a YugabyteDB build:
 
+```
 cd ~/code
 git clone https://github.com/yugabyte/yugabyte-db.git
 cd yugabyte-db
 export YB_THIRDPARTY_DIR=~/code/yugabyte-db-thirdparty
-./yb_build.sh --gcc9
+./yb_build.sh --clang12
 ```
 
 The [ci.yml](https://github.com/yugabyte/yugabyte-db-thirdparty/blob/master/.github/workflows/ci.yml) GitHub Actions workflow file contains multiple operating system and compiler configurations that we build and test regularly. Take a look at the command lines used in that file to get an idea of various usable combinations of parameters to the `build_thirdparty.sh` script.
 
 The third-party dependencies themselves are described, each in its own Python file, in the [build_definitions](https://github.com/yugabyte/yugabyte-db-thirdparty/tree/master/python/build_definitions) directory.
+
+## Modifying build definition for a dependency
+
+To modify some dependency (e.g. glog):
+* Look at the corresponding python file in build_definitions, e.g. https://github.com/yugabyte/yugabyte-db-thirdparty/blob/master/python/build_definitions/glog.py, to see where it is pulling the dependency from. Usually it is a GitHub repo, either the open-source upstream repo or our own fork.
+* In case of glog, it is https://github.com/yugabyte/glog. Fork that repo on GitHub, to e.g. https://github.com/yourgithubusername/glog. Make all the necessary changes in your fork, push changes to a branch on your fork, and create a tag, e.g. `0.4.0-yb-yourusername-1`. Push the tag to your fork.
+* Modify the dependency python file, i.e. glog.py in this case, to reference your own repo and tag:
+
+```python
+        super(GLogDependency, self).__init__(
+            name='glog',
+            version='0.4.0-yb-yourusername-1',
+            url_pattern='https://github.com/yourgithubyusername/glog/archive/v{0}.tar.gz',
+            build_group=BUILD_GROUP_INSTRUMENTED)
+```
+
+* Rebuild thirdparty (`--add-checksum` so it would auto-add the checksum for your archive):
+
+```
+./build_thirdparty.sh --add-checksum
+```
+
+* When you are done with your changes to the dependency, e.g. glog, get them checked into our fork, i.e. yugabyte/glog on GitHub, and create the next "official" Yugabyte tag for that dependency, e.g. 0.4.0-yb-2 (we usually add a `-yb-N` suffix to the upstream version). Then in your PR to yugabyte-db-thirdparty, reference that tag from the dependency's python file as shown above.
+
+## Ensuring a clean build
+
+We don't currently use separate directories for different "build types" in yugabyte-db-thirdparty unlike we do in YugabyteDB. Therefore, when changing to a different compiler type, compiler flags, etc., it would be safest to remove build output before rebuilding:
+
+```
+rm -rf build installed
+```