Auto merge of #14452 - PeterZhizhin:upgrade-pip-with-new-virtualenv, r=frewsxcv

Commit that fixes the issue #11074 by upgrading pip whenever virtuale…

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I have kind of resolved the issue #11074 by adding bool variable which is set to `True` if we had created the virtualenv and `False` otherwise.

Then it updates pip by executing `pip install --upgrade pip` in the same way as packages are updated. I am a little bit worried that I have almost duplicated the installation routine from the `for` loop but I am not sure whether I should add a function or not.

I think it is the best way of doing this because it does not need any Internet access for regular work (only for the first time you execute mach) as @larsbergstrom worried [here](https://github.com/servo/servo/pull/11149). It also doesn't add any extra latency on a no-op build.

I have checked the solution inside a docker container based on debian wheezy. Before the patch `./mach` failed to run because it wasn't able to install some packages. Now it runs successfully.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #11074 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because it changes only mach_bootstrap.py

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python/mach_bootstrap.py

@@ -112,6 +112,7 @@ def _activate_virtualenv(topdir):
 
     script_dir = _get_virtualenv_script_dir()
     activate_path = os.path.join(virtualenv_path, script_dir, "activate_this.py")
+    need_pip_upgrade = False
     if not (os.path.exists(virtualenv_path) and os.path.exists(activate_path)):
         virtualenv = _get_exec_path(VIRTUALENV_NAMES)
         if not virtualenv:
@@ -123,6 +124,8 @@ def _activate_virtualenv(topdir):
             out, err = process.communicate()
             print('Python virtualenv failed to execute properly:')
             sys.exit('Output: %s\nError: %s' % (out, err))
+        # We want to upgrade pip when virtualenv created for the first time
+        need_pip_upgrade = True
 
     execfile(activate_path, dict(__file__=activate_path))
 
@@ -143,6 +146,20 @@ def _activate_virtualenv(topdir):
         os.path.join("tests", "wpt", "harness", "requirements_servo.txt"),
     ]
 
+    if need_pip_upgrade:
+        # Upgrade pip when virtualenv is created to fix the issue
+        # https://github.com/servo/servo/issues/11074
+        pip = _get_exec_path(PIP_NAMES, is_valid_path=check_exec_path)
+        if not pip:
+            sys.exit("Python pip is either not installed or not found in virtualenv.")
+
+        process = Popen([pip, "install", "-q", "-U", "pip"], stdout=PIPE, stderr=PIPE)
+        process.wait()
+        if process.returncode:
+            out, err = process.communicate()
+            print('Pip failed to upgrade itself properly:')
+            sys.exit('Output: %s\nError: %s' % (out, err))
+
     for req_rel_path in requirements_paths:
         req_path = os.path.join(topdir, req_rel_path)
         marker_file = req_rel_path.replace(os.path.sep, '-')