Fix locking to work with flutter and dart running simultaneously (#133350)

bin/dart

@@ -45,6 +45,7 @@ function follow_links() (
 
 PROG_NAME="$(follow_links "${BASH_SOURCE[0]}")"
 BIN_DIR="$(cd "${PROG_NAME%/*}" ; pwd -P)"
+SHARED_NAME="$BIN_DIR/internal/shared.sh"
 OS="$(uname -s)"
 
 # If we're on Windows, invoke the batch script instead to get proper locking.
@@ -53,6 +54,6 @@ if [[ $OS =~ MINGW.* || $OS =~ CYGWIN.* || $OS =~ MSYS.* ]]; then
 fi
 
 # To define `shared::execute()` function
-source "$BIN_DIR/internal/shared.sh"
+source "$SHARED_NAME"
 
 shared::execute "$@"

bin/flutter

@@ -50,6 +50,7 @@ function follow_links() (
 
 PROG_NAME="$(follow_links "${BASH_SOURCE[0]}")"
 BIN_DIR="$(cd "${PROG_NAME%/*}" ; pwd -P)"
+SHARED_NAME="$BIN_DIR/internal/shared.sh"
 OS="$(uname -s)"
 
 # If we're on Windows, invoke the batch script instead to get proper locking.
@@ -58,6 +59,6 @@ if [[ $OS =~ MINGW.* || $OS =~ CYGWIN.* || $OS =~ MSYS.* ]]; then
 fi
 
 # To define `shared::execute()` function
-source "$BIN_DIR/internal/shared.sh"
+source "$SHARED_NAME"
 
 shared::execute "$@"

bin/internal/shared.sh

@@ -229,7 +229,23 @@ function shared::execute() {
     exit 1
   fi
 
-  upgrade_flutter 7< "$PROG_NAME"
+  # File descriptor 7 is prepared here so that we can use it with
+  # flock(1) in _lock() (see above).
+  #
+  # We use number 7 because it's a luckier number than 3; luck is
+  # important when making locks work reliably. Also because that way
+  # if anyone is redirecting other file descriptors there's less
+  # chance of a conflict.
+  #
+  # In any case, the file we redirect into this file descriptor is
+  # this very source file you are reading right now, because that's
+  # the only file we can truly guarantee exists, since we're running
+  # it. We don't use PROG_NAME because otherwise if you run `dart` and
+  # `flutter` simultaneously they'll end up using different lock files
+  # and will corrupt each others' downloads.
+  #
+  # SHARED_NAME itself is prepared by the caller script.
+  upgrade_flutter 7< "$SHARED_NAME"
 
   BIN_NAME="$(basename "$PROG_NAME")"
   case "$BIN_NAME" in