@@ -66,7 +66,8 @@ function pip_install_gr {

 # Wrapper for ``pip install`` to set cache and proxy environment variables

 # Uses globals ``OFFLINE``, ``PIP_VIRTUAL_ENV``,

-# ``PIP_UPGRADE``, ``TRACK_DEPENDS``, ``*_proxy``

+# ``PIP_UPGRADE``, ``TRACK_DEPENDS``, ``*_proxy``,

+# ``USE_CONSTRAINTS``

 # pip_install package [package ...]

 function pip_install {

     local xtrace=$(set +o | grep xtrace)

@@ -103,6 +104,13 @@ function pip_install {

         fi

     fi

+    cmd_pip="$cmd_pip install"

+

+    # Handle a constraints file, if needed.

+    if [[ "$USE_CONSTRAINTS" == "True" ]]; then

+        cmd_pip="$cmd_pip -c $REQUIREMENTS_DIR/upper-constraints.txt"

+    fi

+

     local pip_version=$(python -c "import pip; \

                         print(pip.__version__.strip('.')[0])")

     if (( pip_version<6 )); then

@@ -116,7 +124,7 @@ function pip_install {

         https_proxy="${https_proxy:-}" \

         no_proxy="${no_proxy:-}" \

         PIP_FIND_LINKS=$PIP_FIND_LINKS \

-        $cmd_pip install $upgrade \

+        $cmd_pip $upgrade \

         $@

     # Also install test requirements

@@ -128,7 +136,7 @@ function pip_install {

             https_proxy=${https_proxy:-} \

             no_proxy=${no_proxy:-} \

             PIP_FIND_LINKS=$PIP_FIND_LINKS \

-            $cmd_pip install $upgrade \

+            $cmd_pip $upgrade \

             -r $test_req

     fi

 }

@@ -215,7 +223,7 @@ function setup_package_with_req_sync {

     # ``errexit`` requires us to trap the exit code when the repo is changed

     local update_requirements=$(cd $project_dir && git diff --exit-code >/dev/null || echo "changed")

-    if [[ $update_requirements != "changed" ]]; then

+    if [[ $update_requirements != "changed" && "$USE_CONSTRAINTS" == "False" ]]; then

         if is_in_projects_txt $project_dir; then

             (cd $REQUIREMENTS_DIR; \

                 ./.venv/bin/python update.py $project_dir)

@@ -227,6 +235,12 @@ function setup_package_with_req_sync {

         fi

     fi

+    if [ -n "$REQUIREMENTS_DIR" ]; then

+        # Constrain this package to this project directory from here on out.

+        local name=$(awk '/^name.*=/ {print $3}' $project_dir/setup.cfg)

+        $REQUIREMENTS_DIR/.venv/bin/edit-constraints $REQUIREMENTS_DIR/upper-constraints.txt -- $name "$flags $project_dir"

+    fi

+

     setup_package $project_dir $flags

     # We've just gone and possibly modified the user's source tree in an

@@ -683,6 +683,9 @@ save_stackenv $LINENO

 echo_summary "Installing package prerequisites"

 source $TOP_DIR/tools/install_prereqs.sh

+# Normalise USE_CONSTRAINTS

+USE_CONSTRAINTS=$(trueorfalse False USE_CONSTRAINTS)

+

 # Configure an appropriate Python environment

 if [[ "$OFFLINE" != "True" ]]; then

     PYPI_ALTERNATIVE_URL=${PYPI_ALTERNATIVE_URL:-""} $TOP_DIR/tools/install_pip.sh

@@ -149,6 +149,12 @@ DATABASE_QUERY_LOGGING=$(trueorfalse False DATABASE_QUERY_LOGGING)

 # Zero disables timeouts

 GIT_TIMEOUT=${GIT_TIMEOUT:-0}

+# Constraints mode

+# - False (default) : update git projects dependencies from global-requirements.

+#

+# - True : use upper-constraints.txt to constrain versions of packages intalled

+#          and do not edit projects at all.

+USE_CONSTRAINTS=${USE_CONSTRAINTS:-False}

 # Repositories

 # ------------