Adds a new sage-flock command based on the original pip2-lock command…

…, but generalized to

running any command.

This is like a simplified version of the flock command on some systems, but I don't know how portable
that is (granted, flock(2) isn't necessarily all that portable either).

This removes the pip2-lock and pip3-lock commands and replaces their usage with the general sage-flock
command.

build/bin/sage-pip-install

@@ -60,10 +60,11 @@ fi
 # We should avoid running pip2/3 while uninstalling a package because that
 # is prone to race conditions. Therefore, we use a lockfile while
 # running pip. This is implemented in the Python script pip2/3-lock.
+LOCK="$SAGE_LOCAL/var/lock/$PIP.lock"
 
 # Keep uninstalling as long as it succeeds
 while true; do
-    out=$($PIP-lock uninstall --disable-pip-version-check -y "$name" 2>&1)
+    out=$(sage-flock -x $LOCK $PIP uninstall --disable-pip-version-check -y "$name" 2>&1)
     if [ $? -ne 0 ]; then
         break
     fi
@@ -83,9 +84,8 @@ fi
 # to apply a shared lock)
 echo "Installing package $name using $PIP"
 
-$PIP-lock SHARED install $pip_install_flags .
+sage-flock -s $LOCK $PIP install $pip_install_flags .
 if [ $? -ne 0 ]; then
     echo >&2 "Error: installing with $PIP failed"
     exit 3
 fi
-

build/bin/sage-spkg

@@ -753,7 +753,8 @@ if [ "$UNAME" = "CYGWIN" ]; then
     # Rebase after installing each package--in case any packages load this
     # package at build time we need to ensure during the build that no binaries
     # have conflicting address spaces
-    sage-rebase.sh "$SAGE_LOCAL" 2>/dev/null
+    sage-flock -x "$SAGE_LOCAL/var/lock/rebase.lock" \
+        sage-rebase.sh "$SAGE_LOCAL" 2>/dev/null
 fi
 
 echo "Successfully installed $PKG_NAME"

src/bin/sage-flock

@@ -0,0 +1,73 @@
+#!/usr/bin/env sage-python23
+# vim: set filetype=python:
+"""
+This script runs the given command under a file lock (similar to the flock
+command on some systems).
+
+The command may also be a Python module in which case it is run in the same
+Python interpreter as this script (a small optimization to avoid restarting the
+Python interpreter).
+"""
+
+# This is originally motivated by pip, but has since been generalized.  We
+# should avoid running pip while uninstalling a package because that is prone
+# to race conditions. This script runs pip under a lock.  For details, see
+# https://trac.sagemath.org/ticket/21672
+
+from __future__ import print_function
+
+
+import argparse
+import fcntl
+import os
+import pipes
+import sys
+
+
+def main(argv=None):
+    parser = argparse.ArgumentParser(description=__doc__)
+    group = parser.add_mutually_exclusive_group()
+    group.add_argument('-s', '--shared', action='store_true',
+                       help='create a shared lock')
+
+    # Note: A exclusive lock is created by default if no other flags are given,
+    # but supplying the --exclusive flag explicitly may help clarity
+    group.add_argument('-x', '--exclusive', action='store_true',
+                       help='create an exclusive lock (the default)')
+    parser.add_argument('lock', metavar='LOCK', type=argparse.FileType('w+'),
+                        help='filename of the lock')
+    parser.add_argument('command', metavar='COMMAND', nargs=argparse.REMAINDER,
+                        help='command to run with the lock including any '
+                             'arguments to that command')
+
+    args = parser.parse_args(argv)
+
+    if args.shared:
+        locktype = fcntl.LOCK_SH
+    else:
+        locktype = fcntl.LOCK_EX
+
+    lock = args.lock
+    command = args.command
+
+    # First try a non-blocking lock such that we can give an informative
+    # message while the user is waiting.
+    try:
+        fcntl.flock(lock, locktype | fcntl.LOCK_NB)
+    except IOError:
+        if locktype == fcntl.LOCK_SH:
+            kind = "shared"
+        else:
+            kind = "exclusive"
+
+        print("Waiting for {0} lock to run {1} ... ".format(
+            kind, ' '.join(pipes.quote(arg) for arg in command)), end='',
+            file=sys.stderr)
+        fcntl.flock(lock, locktype)
+        print("ok", file=sys.stderr)
+
+    os.execvp(command[0], command)
+
+
+if __name__ == '__main__':
+    sys.exit(main())