Retrieve commit from pr.sourceSha for PR builds

1 files changed, 9 insertions, 5 deletions

diff --git a/azure-pipelines.yml b/azure-pipelines.yml
index 6b28088..c90ac96 100644
--- a/azure-pipelines.yml
+++ b/azure-pipelines.yml
@@ -49,16 +49,20 @@ jobs:
   - script: |
       REQUEST_URL="https://dev.azure.com/python-discord/${SYSTEM_TEAMPROJECTID}/_apis/build/builds?queryOrder=finishTimeDescending&resultFilter=succeeded&\$top=1&repositoryType=${BUILD_REPOSITORY_PROVIDER}&repositoryId=${BUILD_REPOSITORY_NAME}&branchName=${BUILD_SOURCEBRANCH}&api-version=5.0"
       echo "Retrieving previous build's commit using $REQUEST_URL"
+      RESPONSE="$(curl -sSL "${REQUEST_URL}")"
 
-      PREV_COMMIT="$(curl -sSL "${REQUEST_URL}" | grep -Po '"sourceVersion":"\K.*?[^\\](?=",)')"
-      if [[ -z $PREV_COMMIT ]]; then
-        echo "Could not retrieve the previous build's commit. Falling back to the head of the target branch."
-        PREV_COMMIT="origin/$SYSTEM_PULLREQUEST_TARGETBRANCH"
+      if [[ $BUILD_REASON = "PullRequest" ]]; then
+        PREV_COMMIT="$(echo "${RESPONSE}" | grep -Po '"pr.sourceSha":"\K.*?[^\\](?=",)')"
+        if [[ -z $PREV_COMMIT ]]; then
+          echo "Could not retrieve the previous build's commit. Falling back to the head of the target branch."
+          PREV_COMMIT="origin/$SYSTEM_PULLREQUEST_TARGETBRANCH"
       else
-        echo "Retrieved $PREV_COMMIT as previous build's commit."
+        PREV_COMMIT="$(echo "${RESPONSE}" | grep -Po '"sourceVersion":"\K.*?[^\\](?=",)')"
       fi
 
       if [[ -n $PREV_COMMIT ]]; then
+        echo "Using $PREV_COMMIT to compare diffs."
+
         if [[ -z "$(git diff $PREV_COMMIT -- docker/base.Dockerfile)" ]]; then
           echo "No changes detected in docker/base.Dockerfile. The base image will not be built."
           echo "##vso[task.setvariable variable=BASE_CHANGED]false"