rake db:structure:dump warns when `mysqldump` is not in PATH.

Closes #9518.

The rake task used to fail silently and left an empty `structure.sql`.
It's confusing for users to get to the root of the problem.
The warning message tells them where to look.

3 files changed, 22 insertions, 2 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 105bf6f88a..a7b5ba9f3e 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,11 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Warn when `rake db:structure:dump` with a mysl database and
+    `mysqldump` is not in the PATH or fails.
+    Fixes #9518.
+
+    *Yves Senn*
+
 *   Remove `connection#structure_dump`, which is no longer used. *Yves Senn*
 
 *   Make it possible to execute migrations without a transaction even
diff --git a/activerecord/lib/active_record/tasks/mysql_database_tasks.rb b/activerecord/lib/active_record/tasks/mysql_database_tasks.rb
index 17378969a5..10696258c9 100644
--- a/activerecord/lib/active_record/tasks/mysql_database_tasks.rb
+++ b/activerecord/lib/active_record/tasks/mysql_database_tasks.rb
@@ -57,7 +57,10 @@ module ActiveRecord
         args.concat(["--result-file", "#{filename}"])
         args.concat(["--no-data"])
         args.concat(["#{configuration['database']}"])
-        Kernel.system(*args)
+        unless Kernel.system(*args)
+          $stderr.puts "Could not dump the database structure. "\
+                       "Make sure `mysqldump` is in your PATH and check the command output for warnings."
+        end
       end
 
       def structure_load(filename)
diff --git a/activerecord/test/cases/tasks/mysql_rake_test.rb b/activerecord/test/cases/tasks/mysql_rake_test.rb
index 38b9dd02f0..dadcca5b7f 100644
--- a/activerecord/test/cases/tasks/mysql_rake_test.rb
+++ b/activerecord/test/cases/tasks/mysql_rake_test.rb
@@ -249,10 +249,21 @@ module ActiveRecord
 
     def test_structure_dump
       filename = "awesome-file.sql"
-      Kernel.expects(:system).with("mysqldump", "--result-file", filename, "--no-data", "test-db")
+      Kernel.expects(:system).with("mysqldump", "--result-file", filename, "--no-data", "test-db").returns(true)
 
       ActiveRecord::Tasks::DatabaseTasks.structure_dump(@configuration, filename)
     end
+
+    def test_warn_when_external_structure_dump_fails
+      filename = "awesome-file.sql"
+      Kernel.expects(:system).with("mysqldump", "--result-file", filename, "--no-data", "test-db").returns(false)
+
+      warnings = capture(:stderr) do
+        ActiveRecord::Tasks::DatabaseTasks.structure_dump(@configuration, filename)
+      end
+
+      assert_match(/Could not dump the database structure/, warnings)
+    end
   end
 
   class MySQLStructureLoadTest < ActiveRecord::TestCase