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[lldb/Target] Rework the way the inferior environment is created
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Authored by friss on Mar 19 2020, 6:02 PM.

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Description

[lldb/Target] Rework the way the inferior environment is created

Summary:
The interactions between the environment settings (target.env-vars,
target.inherit-env) and the inferior life-cycle are non-obvious
today. For example, if target.inherit-env is set, the target.env-vars
setting will be augmented with the contents of the host environment
the first time the launch environment is queried (usually at
launch). After that point, toggling target.inherit-env will have no
effect as there's no tracking of what comes from the host and what is
a user setting.

This patch computes the environment every time it is queried rather
than updating the contents of the target.env-vars property. This
means that toggling the target.inherit-env property later will now
have the intended effect.

This patch also adds a target.unset-env-vars settings that one can
use to remove variables from the launch environment. Using this, you
can inherit all but a few of the host environment.

The way the launch environment is constructed is:

1/ if `target.inherit-env` is set, then read the host environment
into the launch environment.
2/ Remove for the environment the variables listed in
`target.unset-env`.
3/ Augment the launch environment with the contents of
`target.env-vars`. This overrides any common values with the host
environment.

The one functional difference here that could be seen as a regression
is that target.env-vars will not contain the inferior environment
after launch. The patch implements a better alternative in the
target show-launch-environment command which will return the
environment computed through the above rules.

Reviewers: labath, jingham

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D76470

(cherry picked from commit b4a6e63ea12309bf667d1569a20ec5b081cbf2a4)