Sometimes problems are too specific, but they can show us a path to look for a more general problem and its solution. There was a nasty bug in emerging Firefox package in Gentoo basically it compiled all the source and then it got an error from the build script, but all the source was compiled successfully! So the solution was just to manually install with

ebuild

the package and not to wait for the fix 😉 A good workaround.

But what if we have a big package, which failed during compilation

because of “out of ram” or “out of space” or a missing library, which the maintainer did not included in the dependencies. So tens of minutes or even hours of compilation (yes, there are still such packages like chromium) is wasted and we must start up from the beginning? No you can continue the current failed compilation from the exact point of failure using “ebuild”.
In our example we have a failed compilation of chromium with “out of memory”. We need the exact version of the package, scroll to your emerge command and copy the version, in our case it was: chromium-67.0.3377.1.ebuild
Here are the commands:

STEP 1) Continue compilation

[root@local ]# ebuild /usr/portage/www-client/chromium/chromium-67.0.3377.1.ebuild compile
>>> Existing ${T}/environment for 'chromium-67.0.3377.1' will be sourced.
>>> Run 'clean' to start with a fresh environment.
>>> Checking chromium-67.0.3377.1.tar.xz's mtime...
>>> WORKDIR is up-to-date, keeping...
 * checking ebuild checksums ;-) ...                                                                                                    [ ok ]
 * checking auxfile checksums ;-) ...                                                                                                   [ ok ]
 * checking miscfile checksums ;-) ...                                                                                                  [ ok ]
 * Checking for at least 3 GiB RAM ...                                                                                                  [ ok ]
 * Checking for at least 5 GiB disk space at "/var/tmp/portage/www-client/chromium-67.0.3377.1/temp" ...                                [ ok ]
>>> It appears that 'setup' has already executed for 'chromium-67.0.3377.1'; skipping.
>>> Remove '/var/tmp/portage/www-client/chromium-67.0.3377.1/.setuped' to force setup.
>>> It appears that 'unpack' has already executed for 'chromium-67.0.3377.1'; skipping.
>>> Remove '/var/tmp/portage/www-client/chromium-67.0.3377.1/.unpacked' to force unpack.
>>> It appears that 'prepare' has already executed for 'chromium-67.0.3377.1'; skipping.
>>> Remove '/var/tmp/portage/www-client/chromium-67.0.3377.1/.prepared' to force prepare.
>>> It appears that 'configure' has already executed for 'chromium-67.0.3377.1'; skipping.
>>> Remove '/var/tmp/portage/www-client/chromium-67.0.3377.1/.configured' to force configure.
>>> Compiling source in /var/tmp/portage/www-client/chromium-67.0.3377.1/work/chromium-67.0.3377.1 ...
ninja -v -j6 -l6 -C out/Release mksnapshot
ninja: Entering directory `out/Release'

As you can see in the output above an existing environment for ‘chromium-67.0.3377.1’ will be sourced.
Here the compilation continue from the last failed compilation script, it skipped multiple source dependencies.

STEP 2) Install the package

In fact two commands:

ebuild /usr/portage/www-client/chromium/chromium-67.0.3377.1.ebuild install
ebuild /usr/portage/www-client/chromium/chromium-67.0.3377.1.ebuild qmerge

The first command “install” will install the package in the working directory of the emerge process and then the second “qmerge” will install all the files of the package in the install directory to the live filesystem and will do some additional checks and modifications in your systems package database to install the package properly as if the emerge was used.