Testing PostgreSQL extensions on Travis CI

I have cobbled together some scripts to be able to test PostgreSQL extensions against multiple PostgreSQL major versions on Travis CI. (This requires that the extension is hosted on GitHub.) See the configuration for PL/sh and the build output as examples. Perhaps others will find this useful for their extensions as well.

Tricky shell local variables

I have a word of warning against improper use of local in shell functions.

If you are using shell functions, you might want to declare some variables local to the shell function. That is good. The basic syntax for that is

local a b c

In some shells, you can also combine the local declaration and assignment, like this:

local foo=$1
local bar=$2

(The Debian policy even explicitly allows it.)

This is somewhat dangerous.

Bare shell assignment like

foo=$bar

does not perform word splitting, so the above is safe even if there are spaces in $bar. But the local command does perform word splitting (because it can take multiple arguments, as in the first example), so the seemingly similar

local foo=$bar

is not safe.

This can be really confusing when you add local to existing code and it starts breaking.

You can avoid this, of course, by always quoting everything to like

local foo="$bar"

but overquoting isn't always desirable, because it can make code less readable when commands are nested, like

local foo="$(otherfunc "other arg")"

(Nesting is legal and works fine in this case, however.)

I suggest using local only for declaring variables, and using separate assignment statements. That way, all assignments are parsed in the same way.