As discussed in https://github.com/containers/podman/issues/20000, we
need an opt-in mechanism to _append_ string arrays during loading
sequence of containers.conf files.
At the moment, existing fields/data will be overriden with each loaded
config that sets the specified field/option. The TOML (toml.io) config
format does not allow for attributing fields and structs are implicitly
represented as "tables". I wanted to extend a string array with a
simple boolean field, for instance:
```TOML
env=["FOO=bar"]
env.append=true
```
TOML doesn't suppor tthe upper idea as it's not a properly formatted
table. So I looked for alternatives and found that TOML supports
so-called "mixed-type arrays". As the same suggests, such arrays allow
for including more than one type and that seemed like a reasonable
candidate as it allows for _extending_ the existing syntax without
introducing new fields or even yet-another way of loading conf files.
The new format can be seen in the tests. Please note that this is just
a _tested_ POC. Integrating the POC in containers.conf may turn into a
bigger journey as Podman is directly (ab)using many of the fields.
Since they have to be changed to the new type (see POC), Podman will not
compile without changes.
Signed-off-by: Valentin Rothberg <vrothberg@redhat.com>
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