[processing] Fix GDAL algorithms hang when gdal command is not availa…

…ble to run

python/core/auto_generated/qgsrunprocess.sip.in

@@ -129,6 +129,13 @@ After execution completes, the process' result code will be returned.
     QProcess::ExitStatus exitStatus() const;
 %Docstring
 After a call to :py:func:`~QgsBlockingProcess.run`, returns the process' exit status.
+%End
+
+    QProcess::ProcessError processError() const;
+%Docstring
+After a call to :py:func:`~QgsBlockingProcess.run`, returns the process' reported error.
+
+Returns QProcess.UnknownError if no error occurred.
 %End
 
 };

python/plugins/processing/algs/gdal/GdalUtils.py

@@ -150,6 +150,8 @@ def on_stderr(ba):
             raise QgsProcessingException(GdalUtils.tr('Process was unexpectedly terminated'))
         elif res == 0:
             feedback.pushInfo(GdalUtils.tr('Process completed successfully'))
+        elif proc.processError() == QProcess.FailedToStart:
+            raise QgsProcessingException(GdalUtils.tr('Process {} failed to start. Either {} is missing, or you may have insufficient permissions to run the program.').format(command, command))
         else:
             feedback.reportError(GdalUtils.tr('Process returned error code {}').format(res))
 

src/core/qgsrunprocess.cpp

@@ -272,8 +272,9 @@ int QgsBlockingProcess::run( QgsFeedback *feedback )
 
   int result = 0;
   QProcess::ExitStatus exitStatus = QProcess::NormalExit;
+  QProcess::ProcessError error = QProcess::UnknownError;
 
-  std::function<void()> runFunction = [ this, &result, &exitStatus, feedback]()
+  std::function<void()> runFunction = [ this, &result, &exitStatus, &error, feedback]()
   {
     // this function will always be run in worker threads -- either the blocking call is being made in a worker thread,
     // or the blocking call has been made from the main thread and we've fired up a new thread for this function
@@ -319,8 +320,16 @@ int QgsBlockingProcess::run( QgsFeedback *feedback )
       mStderrHandler( ba );
     } );
     p.start( mProcess, mArguments, QProcess::Unbuffered | QProcess::ReadWrite );
-
-    loop.exec();
+    if ( !p.waitForStarted() )
+    {
+      result = 1;
+      exitStatus = QProcess::NormalExit;
+      error = p.error();
+    }
+    else
+    {
+      loop.exec();
+    }
 
     mStdoutHandler( p.readAllStandardOutput() );
     mStderrHandler( p.readAllStandardError() );
@@ -339,11 +348,17 @@ int QgsBlockingProcess::run( QgsFeedback *feedback )
   }
 
   mExitStatus = exitStatus;
+  mProcessError = error;
   return result;
 }
 
 QProcess::ExitStatus QgsBlockingProcess::exitStatus() const
 {
   return mExitStatus;
 };
+
+QProcess::ProcessError QgsBlockingProcess::processError() const
+{
+  return mProcessError;
+};
 #endif // QT_CONFIG(process)

src/core/qgsrunprocess.h

@@ -185,6 +185,13 @@ class CORE_EXPORT QgsBlockingProcess : public QObject
      */
     QProcess::ExitStatus exitStatus() const;
 
+    /**
+     * After a call to run(), returns the process' reported error.
+     *
+     * Returns QProcess::UnknownError if no error occurred.
+     */
+    QProcess::ProcessError processError() const;
+
   private:
 
     QString mProcess;
@@ -193,6 +200,7 @@ class CORE_EXPORT QgsBlockingProcess : public QObject
     std::function< void( const QByteArray & ) > mStderrHandler;
 
     QProcess::ExitStatus mExitStatus = QProcess::NormalExit;
+    QProcess::ProcessError mProcessError = QProcess::UnknownError;
 };
 
 #endif // QT_CONFIG(process)

tests/src/python/test_qgsblockingprocess.py

@@ -113,6 +113,31 @@ def std_err(ba):
         self.assertNotEqual(p.run(f), 0)
         self.assertEqual(p.exitStatus(), QProcess.CrashExit)
 
+    def test_process_no_file(self):
+        """
+        Test a script which doesn't exist
+        """
+
+        def std_out(ba):
+            std_out.val += ba.data().decode('UTF-8')
+
+        std_out.val = ''
+
+        def std_err(ba):
+            std_err.val += ba.data().decode('UTF-8')
+
+        std_err.val = ''
+
+        # this program definitely doesn't exist!
+        p = QgsBlockingProcess('qgis_sucks', ['--version'])
+        p.setStdOutHandler(std_out)
+        p.setStdErrHandler(std_err)
+
+        f = QgsFeedback()
+        self.assertEqual(p.run(f), 1)
+        self.assertEqual(p.exitStatus(), QProcess.NormalExit)
+        self.assertEqual(p.processError(), QProcess.FailedToStart)
+
     def test_process_env(self):
         """
         Test that process inherits system environment correctly