Plugins installed outside of the plugin browser are showing an undefined index error

[sswebster]

When installing plugins outside of the WordPress plugin browser with debugging enabled, I'm receiving a "Notice: Undefined Index: Process_success_specific" error. This doesn't happen when installing a plugin through the plugin browser using ajax, but does happen if you install a plugin by navigating to the install URL. It looks to be related to an upgrader string added in this commit: ​https://github.com/WordPress/WordPress/commit/49bbff551ba5d8c6e9502835a9dcd466b6585406#diff-572613cbd6b420aa292bf4dab1a5f6e5

[sswebster]

Here is a screenshot of the issue when installing the Gutenberg plugin

This PR fixes an undefined index error that occurs when attempting to access the process_success_specific key. The fix prepopulates the WP_Upgrader->strings array, ensuring the process_success_specific key exists.

Trac ticket: https://core.trac.wordpress.org/ticket/50837

[nateinaction]

This patch is my first attempt to contribute to core. All feedback welcome.

[SergeyBiryukov]

Just noting that I was able to reproduce the issue, but it's not quite clear why this only happens for plugins and not for themes.

[SergeyBiryukov]

Ah, it's because the Theme_Upgrader class has the string in the ::install_strings() method, while Plugin_Upgrader has it in ::upgrade_strings().

Moving the string to the correct method appears to resolve the issue.

[SergeyBiryukov]

In 48734:

Plugins: Move the "Successfully installed the plugin" string to the correct Plugin_Upgrader method.

The string refers to the installation, and belongs in the ::install_strings() method, not in ::upgrade_strings().

Props sswebster, nateinaction, SergeyBiryukov.
Fixes #50837.

[SergeyBiryukov]

Reopening for the 5.5 branch.

[SergeyBiryukov]

In 48735:

Plugins: Move the "Successfully installed the plugin" string to the correct Plugin_Upgrader method.

The string refers to the installation, and belongs in the ::install_strings() method, not in ::upgrade_strings().

Props sswebster, nateinaction.
Reviewed by whyisjake, SergeyBiryukov.
Merges [48734] to the 5.5 branch.
Fixes #50837.

Merged in https://core.trac.wordpress.org/changeset/48735.