Sharing configuration between branches in appveyor.yml

The problem

There are two options to have branch-specific configuration with appveyor.yml:

Commit appveyor.yml into each branch with branch-specific settings.
Use Conditional build configuration where appveyor.yml has a list of configurations for different branches.

The problem with 1st approach is merging as you are overriding appveyor.yml in the base branch with one from the branch being merged.

2nd approach requires appveyor.yml of the following format:

# configuration for "master" branch
# build in Release mode and deploy to Azure
-
  branches:
    only:
      - master

  configuration: Release

  deploy:
    provider: AzureCS
    ...

# configuration for all branches starting from "dev-"
# build in Debug mode and deploy locally for testing
-
  branches:
    only:
      - /dev-.*/

  configuration: Debug

  deploy:
    provider: Local
    ...

# "fall back" configuration for all other branches
# no "branches" section defined
# do not deploy at all
-
  configuration: Debug

While this approach works great in the most cases there is one incovenience though - with large configuration and many branches appveyor.yml becomes really unmanageable and error-prone as you have to repeat (copy-paste) entire configuration for every branch.

The solution

We just deployed an update to AppVeyor that allows sharing common configuration between branches in a single appveyor.yml!

There is new for node with a list of branch-specific configurations overriding common configuration defined on the top most level, for example:

# common configuration for ALL branches
environment:
  MY_VAR1: value-A

init:
- do_something_on_init.cmd

install:
- do_something_on_install.cmd

configuration: Debug

# here we are going to override common configuration
for:

# override settings for `master` branch
-
  branches:
    only:
      - master

  configuration: Release

  deploy:
    provider: FTP
    ...

# override settings for `dev-*` branches
-
  branches:
    only:
      - /dev-.*/

  environment:
    MY_VAR2: value-B

  deploy:
    provider: Local
    ...

In the example above we define environment, init and install sections for all branches as well as stating that default configuration is Debug. Then, for master branch we override default settings by changing configuration to Release and adding deployment with FTP provider. For dev-* branches we define a second environment variable MY_VAR2 and enable deployment to Local environment.

Configuration merging rules:

Scalar values such as image, version, configuration, platform, etc. defined on branch level override default ones;
Script sections such init, install, before_build, test_script, etc. defined on branch level override default ones;
Environment variables defined in environment sections are merged (new) and overridden (existing);
Build matrix defined on branch level merges with default one;
deploy, artifacts, notifications section can be either overridden or extended.

For example, consider the following configuration:

artifacts:
- path: bin

deploy:
- provider: Local
  ...

notifications:
- provider: Email
  ...

for:
  branches:
    only:
      - master

  artifacts:
  - path: docs

  deploy: off

  notifications:
    provider: Slack
    ...

In the example above we do the following:

For master branch we adding docs folder to artifacts definition, so both bin and docs folders collected. Both default and branch-specific collections were merged.
For master branch we disable any deployment. off or none on branch-level clears default collection.
For master branch we replace all notifications on default level with a single Slack notification.

Best regards,
AppVeyor team