Use QPDF_EXECUTABLE as a hint for completion

ChangeLog

@@ -1,3 +1,11 @@
+2020-03-31  Jay Berkenbilt  <ejb@ql.org>
+
+	* If QPDF_EXECUTABLE is set, use it as the path to qpdf for
+	purposes of completion. This variable is only read during the
+	executation of `qpdf --completion-zsh` and `qpdf
+	--completion-bash`. It is not used during the actual evaluation of
+	completions.
+
 2020-02-22  Jay Berkenbilt  <ejb@ql.org>
 
 	* Update pdf-set-form-values.cc to use and mention

manual/qpdf-manual.xml

@@ -593,6 +593,14 @@ make
     path, it will warn you, and the completion won't work if you're in
     a different directory.
    </para>
+   <para>
+    qpdf will use <literal>argv[0]</literal> to figure out where its
+    executable is. This may produce unwanted results in some cases,
+    especially if you are trying to use completion with copy of qpdf
+    that is built from source. You can specify a full path to the qpdf
+    you want to use for completion in the
+    <literal>QPDF_EXECUTABLE</literal> environment variable.
+   </para>
   </sect1>
   <sect1 id="ref.basic-options">
    <title>Basic Options</title>

qpdf/qpdf.cc

@@ -1599,12 +1599,17 @@ void
 ArgParser::argCompletionBash()
 {
     std::string progname = argv[0];
-    // Detect if we're in an AppImage and adjust
+    std::string executable;
     std::string appdir;
     std::string appimage;
-    if (QUtil::get_env("APPDIR", &appdir) &&
-        QUtil::get_env("APPIMAGE", &appimage))
+    if (QUtil::get_env("QPDF_EXECUTABLE", &executable))
     {
+        progname = executable;
+    }
+    else if (QUtil::get_env("APPDIR", &appdir) &&
+             QUtil::get_env("APPIMAGE", &appimage))
+    {
+        // Detect if we're in an AppImage and adjust
         if ((appdir.length() < strlen(argv[0])) &&
             (strncmp(appdir.c_str(), argv[0], appdir.length()) == 0))
         {