Skip substitution when the symlink is correct. - #3193
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In google#3192 we ensure that fetched files are modified during fetch, in order to front-load modifications of fetched resources. Substitution also requires modification of the fetched tree. cvd fetch and cvd start can both trigger substitution -- we require both since we need to operate on fetched and local builds. However, we try to modify the tree during cvd start as well, even when the symlinks are already correct. If we have made the host artefacts read-only after fetching them, the subsequent substitution fails. There's no need to regenerate the symlink; do it once and only once.
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In #3192 we ensure that fetched files are modified during fetch, in order to front-load modifications of fetched resources. Substitution also requires modification of the fetched tree. cvd fetch and cvd start can both trigger substitution -- we require both since we need to operate on fetched and local builds.
However, we try to modify the tree during cvd start as well, even when the symlinks are already correct. If we have made the host artefacts read-only after fetching them, the subsequent substitution fails. There's no need to regenerate the symlink; do it once and only once.