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| -rw-r--r-- | doc/frontend.txt | 24 |
1 files changed, 24 insertions, 0 deletions
diff --git a/doc/frontend.txt b/doc/frontend.txt new file mode 100644 index 0000000..9ea1e20 --- /dev/null +++ b/doc/frontend.txt @@ -0,0 +1,24 @@ +Frontend Interfaces +=================== + +This file will try to document some of the interfaces that dpkg makes +available to frontends or that expects them to use, which are currently +not covered by any other type of documentation. + + +Database Locking +---------------- + +Any frontend needing to make sure no write operation is currently happening, +and no other frontend is running should first acquire the frontend lock at +«<admindir>/lock-frontend», and then acquire the dpkg database lock at +«<admindir>/lock». When the frontend invokes dpkg, it should set the +environment variable DPKG_FRONTEND_LOCKED (to prevent dpkg from acquiring +the frontend lock), and then release the dpkg database lock, which will be +acquired by dpkg itself. This way no other frontend following this protocol +can race to perform operations while another one has one in progress. + +These locks must be file record locks (i.e. fcntl(2) advisory locking), and +the whole file should be locked, as that's the most portable way to perform +this operation; this can be achieved by using start=0, len=0 and +whence=SEEK_SET. |