doc: Document branching strategy

Co-authored-by: Brent Watkins <brent.watkins@intel.com>
Signed-off-by: John Andersen <johnandersenpdx@gmail.com>

doc/branching_strategy.md

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+# Branching Strategy
+
+This document is purely for educational purposes to better understand the
+rational behind when new release branches are created and how and when patches
+are applied to them.
+
+Several senarios are covered to help the reader understand when new release get
+created from existing branches versus when new releases get created from new
+branches.
+
+## NB
+
+The version numbers used in this document are for example
+purposes only, please see offical guidance for that the current
+supported branches are.
+
+## References
+
+- https://trunkbaseddevelopment.com/branch-for-release/
+
+## Basic release flow
+
+No users, no bugs, just incrementing off main branch
+
+```mermaid
+graph TD
+  subgraph users[Users]
+    user_a[User A]
+    user_b[User B]
+  end
+
+  subgraph source_branches[Branches in Git repo]
+    subgraph main_history
+      a27dbae3[a27dbae3 - feat]
+      e13eb557[e13eb557 - fix]
+      d18973ae[d18973ae - feat]
+      main[Current HEAD of main]
+
+      a27dbae3 --> e13eb557
+      e13eb557 --> d18973ae
+      d18973ae --> main
+    end
+  end
+
+  subgraph release_tags[Release tags]
+    subgraph release_tag_branch_1_x[Release tags from 1.x branch]
+      subgraph release_tag_branch_1_0_x[Release tags from 1.0.x branch]
+        1_0_0[v1.0.0]
+        1_0_1[v1.0.1]
+      end
+      subgraph release_tag_branch_1_1_x[Release tags from 1.1.x branch]
+        1_1_0[v1.1.0]
+      end
+    end
+  end
+
+  subgraph release_branches[Release branches]
+    subgraph release_branch_1_x[Release branch 1.x]
+      subgraph release_branch_1_0_x[Release branch 1.0.x]
+        release_branch_1_0_x_a27dbae3[a27dbae3]
+        release_branch_1_0_x_e13eb557[e13eb557]
+
+        release_branch_1_0_x_a27dbae3 --> release_branch_1_0_x_e13eb557
+
+        release_branch_1_0_x_a27dbae3 --> 1_0_0
+        release_branch_1_0_x_e13eb557 --> 1_0_1
+      end
+
+      subgraph release_branch_1_1_x[Release branch 1.1.x]
+        release_branch_1_1_x_d18973ae[d18973ae]
+
+        release_branch_1_0_x_e13eb557 --> release_branch_1_1_x_d18973ae
+
+        release_branch_1_1_x_d18973ae --> 1_1_0
+      end
+    end
+  end
+
+  1_0_1 --> user_a
+  1_1_0 --> user_b
+
+  a27dbae3 --> release_branch_1_0_x_a27dbae3
+  e13eb557 --> release_branch_1_0_x_e13eb557
+  d18973ae --> release_branch_1_1_x_d18973ae
+```
+
+## Bugfix in older supported branch
+
+Two supported branches. 1.0.x and 1.1.x. User A is using
+1.0.1 and finds a bug, the bug is no longer present in main
+due to a refactor from introducing a feature (`d18973ae`) causing it
+to not have been present in 1.1.0.
+
+The user reports the bug, we root cause and implement a fix,
+we apply the fix to the 1.0.x branch creating tag v1.0.2.
+
+```mermaid
+graph TD
+  subgraph users[Users]
+    user_a[User A]
+    user_b[User B]
+  end
+
+  subgraph bugs[Bugs]
+    subgraph null_pointer_dereference[Null Pointer Dereference]
+      null_pointer_dereference_found_in[Found]
+      null_pointer_dereference_fix_in[Fixed]
+
+      null_pointer_dereference_found_in --> null_pointer_dereference_fix_in
+    end
+  end
+
+  subgraph source_branches[Branches in Git repo]
+    subgraph main_history
+      a27dbae3[a27dbae3 - feat]
+      e13eb557[e13eb557 - fix]
+      d18973ae[d18973ae - feat]
+      main[Current HEAD of main]
+
+      a27dbae3 --> e13eb557
+      e13eb557 --> d18973ae
+      d18973ae --> main
+    end
+  end
+
+  subgraph release_tags[Release tags]
+      subgraph release_tag_branch_1_0_x[Release tags from 1.0.x branch]
+        1_0_0[v1.0.0]
+        1_0_1[v1.0.1]
+        1_0_2[v1.0.2]
+      end
+      subgraph release_tag_branch_1_1_x[Release tags from 1.1.x branch]
+        1_1_0[v1.1.0]
+      end
+  end
+
+  subgraph release_branches[Release branches]
+      subgraph release_branch_1_0_x[Release branch 1.0.x]
+        release_branch_1_0_x_a27dbae3[a27dbae3]
+        release_branch_1_0_x_e13eb557[e13eb557]
+        release_branch_1_0_x_g47ae21d[g47ae21d]
+
+        release_branch_1_0_x_a27dbae3 --> release_branch_1_0_x_e13eb557
+        release_branch_1_0_x_e13eb557 --> release_branch_1_0_x_g47ae21d
+
+        release_branch_1_0_x_a27dbae3 --> 1_0_0
+        release_branch_1_0_x_e13eb557 --> 1_0_1
+        release_branch_1_0_x_g47ae21d --> 1_0_2
+      end
+
+      subgraph release_branch_1_1_x[Release branch 1.1.x]
+        release_branch_1_1_x_d18973ae[d18973ae]
+
+        release_branch_1_0_x_e13eb557 --> release_branch_1_1_x_d18973ae
+
+        release_branch_1_1_x_d18973ae --> 1_1_0
+      end
+  end
+
+  a27dbae3 --> release_branch_1_0_x_a27dbae3
+  e13eb557 --> release_branch_1_0_x_e13eb557
+  d18973ae --> release_branch_1_1_x_d18973ae
+
+  1_0_1 -->|Containing null pointer dereference| user_a
+  1_1_0 --> user_b
+
+  user_a -->|Using v1.0.1| null_pointer_dereference_found_in
+  null_pointer_dereference_fix_in --> release_branch_1_0_x_g47ae21d
+
+  1_0_2 -->|Fix Null Pointer Dereference| user_a
+```
+
+
+## New minor release
+
+We have two branches we are supporting
+
+- `1.1.x` is the latest and greatest. We recently saw the `v1.1.0` commit cut
+  from `main~1` (`d18973ae`). A recent user, User B, has been using this
+  `v1.1.0` release.
+
+- `1.0.x` this is our stable release branch. Our initial user, User A, has been
+  using this `1.0` minor version due to their usage of an unexposed API which
+  was removed in main during a refactor while introducing a feature
+  (`d18973ae`). We care about keeping them on as a user and therefore we
+  continue to support the `1.0` branch.
+
+User B finds a bug in `v1.1.0` which was determinted to be fixed within the
+refactor that happened when we introduced a new feature in the most recent
+commit to main (`e7612ba4`). We cannot create a `v1.1.1` because something we
+did in the refactor while introducing the latest new feature in main made it so
+that we are unable to reproduce the bug. Therefore, we cannot create a fix and
+cherry-pick it to the `1.1.x` branch.
+
+We are faced with a choice about if we want to support yet another one-off
+branch for User B, similarly to what we are doing with User A, where we made
+them a patch (`g47ae21d`) to fix an issue that was only in the branch their
+releases come from (`1.0.x`) and had been resolved in main. Or if we want to cut
+a new minor release branch off of main.
+
+We talk to User B and they inform us they are not using any internal APIs. This
+means that both our project and User B understand that when we follow Semantic
+Versioning, so long as users stick to exposed API calls, we won't make changes
+between minor versions which will break their calling of our exposed APIs.
+
+We proceed to declare the `1.1.x` branch unsupported, and cut the `1.2.x`
+branch off of `main` at `e7612ba4`. User B then updates their usage to pull in
+the `v1.2.0` version of our software.
+
+```mermaid
+graph TD
+  subgraph users[Users]
+    user_a[User A]
+    user_b[User B]
+  end
+
+  subgraph bugs[Bugs]
+    subgraph null_pointer_dereference[Null Pointer Dereference]
+      null_pointer_dereference_found_in[Found]
+      null_pointer_dereference_fix_in[Fixed]
+
+      null_pointer_dereference_found_in --> null_pointer_dereference_fix_in
+    end
+    subgraph input_validation[Input Validation]
+      input_validation_found_in[Found]
+    end
+  end
+
+  subgraph source_branches[Branches in Git repo]
+    subgraph main_history
+      a27dbae3[a27dbae3 - feat]
+      e13eb557[e13eb557 - fix]
+      d18973ae[d18973ae - feat]
+      e7612ba4[e7612ba4 - feat]
+      main[Current HEAD of main]
+
+      a27dbae3 --> e13eb557
+      e13eb557 --> d18973ae
+      d18973ae --> e7612ba4
+      e7612ba4 --> main
+    end
+  end
+
+  subgraph release_tags[Release tags]
+    subgraph release_tag_branch_1_0_x[Release tags from 1.0.x branch]
+      1_0_0[v1.0.0]
+      1_0_1[v1.0.1]
+      1_0_2[v1.0.2]
+    end
+    subgraph release_tag_branch_1_1_x[Release tags from 1.1.x branch]
+      1_1_0[v1.1.0]
+    end
+    subgraph release_tag_branch_1_2_x[Release tags from 1.2.x branch]
+      1_2_0[v1.2.0]
+    end
+  end
+
+  subgraph release_branches[Release branches]
+    subgraph release_branch_1_0_x[Release branch 1.0.x]
+      release_branch_1_0_x_a27dbae3[a27dbae3]
+      release_branch_1_0_x_e13eb557[e13eb557]
+      release_branch_1_0_x_g47ae21d[g47ae21d]
+
+      release_branch_1_0_x_a27dbae3 --> release_branch_1_0_x_e13eb557
+      release_branch_1_0_x_e13eb557 --> release_branch_1_0_x_g47ae21d
+
+      release_branch_1_0_x_a27dbae3 --> 1_0_0
+      release_branch_1_0_x_e13eb557 --> 1_0_1
+      release_branch_1_0_x_g47ae21d --> 1_0_2
+    end
+
+    subgraph release_branch_1_1_x[Release branch 1.1.x]
+      release_branch_1_1_x_d18973ae[d18973ae]
+
+      release_branch_1_0_x_e13eb557 --> release_branch_1_1_x_d18973ae
+
+      release_branch_1_1_x_d18973ae --> 1_1_0
+    end
+
+    subgraph release_branch_1_2_x[Release branch 1.2.x]
+      release_branch_1_2_x_e7612ba4[e7612ba4]
+
+      release_branch_1_1_x_d18973ae --> release_branch_1_2_x_e7612ba4
+      e7612ba4 --> release_branch_1_2_x_e7612ba4
+
+      release_branch_1_2_x_e7612ba4 --> 1_2_0
+    end
+  end
+
+  a27dbae3 --> release_branch_1_0_x_a27dbae3
+  e13eb557 --> release_branch_1_0_x_e13eb557
+  d18973ae --> release_branch_1_1_x_d18973ae
+
+  1_0_1 -->|Containing null pointer dereference| user_a
+  1_1_0 -->|Containing input validation issue| user_b
+
+  user_a -->|Using v1.0.1| null_pointer_dereference_found_in
+  null_pointer_dereference_fix_in --> release_branch_1_0_x_g47ae21d
+
+  user_b -->|Using v1.1.0| input_validation_found_in
+  input_validation_found_in -->|Fixed during refactor from| e7612ba4
+
+  1_0_2 -->|Fix Null Pointer Dereference| user_a
+  1_2_0 -->|Fix Input Validation issue| user_b
+```