Make missing substitution logic common.

We previously updated part of the substitution logic to not treat
missing files as an error in #1716, but that change only updates the
marker file-based substitutions. To bring this logic to flag-based
substitution, we make the newer logic common to both branches, which
necessitated moving some of the logic around.

Author: 3405691582

base/cvd/cuttlefish/host/commands/cvd/fetch/substitute.cc

@@ -39,6 +39,22 @@
 namespace cuttlefish {
 namespace {
 
+Result<void> Substitute(const std::string& target,
+                        const std::string& full_link_name) {
+  if (!FileExists(target)) {
+    LOG(WARNING) << "Target file " << target << " missing; not making "
+                 << "substitution " << target << " to " << full_link_name;
+    return {};
+  }
+
+  if (FileExists(full_link_name)) {
+    CF_EXPECTF(unlink(full_link_name.c_str()) == 0, "{}", StrError(errno));
+  }
+
+  CF_EXPECT(Symlink(target, full_link_name));
+  return {};
+}
+
 Result<void> SubstituteWithFlag(
     const std::string& target_dir,
     const std::vector<std::string>& host_substitutions) {
@@ -93,11 +109,7 @@ Result<void> SubstituteWithFlag(
     for (const std::string& substitution : host_substitutions) {
       std::string source = fmt::format("{}/{}", bin_dir_parent, substitution);
       std::string to_substitute = fmt::format("{}/{}", target_dir, substitution);
-      // TODO: schuffelen - relax this check after migration completes
-      CF_EXPECTF(FileExists(to_substitute),
-                 "Cannot substitute '{}', does not exist", to_substitute);
-      CF_EXPECTF(unlink(to_substitute.c_str()) == 0, "{}", StrError(errno));
-      CF_EXPECT(Symlink(source, to_substitute));
+      CF_EXPECT(Substitute(source, to_substitute));
     }
   }
 
@@ -118,29 +130,6 @@ Result<std::string> GetCuttlefishCommonDir() {
   return cvd_exe.substr(0, cvd_exe.size() - std::string("/bin/cvd").size());
 }
 
-Result<void> Substitute(const std::string& target_dir,
-                        const std::string& link_name) {
-  static const std::string common_dir = CF_EXPECT(GetCuttlefishCommonDir());
-  std::string target = fmt::format("{}/{}", common_dir, link_name);
-  std::string full_link_name = fmt::format("{}/{}", target_dir, link_name);
-
-  if (!FileExists(target)) {
-     LOG(WARNING) << "Target file " << target << "missing; not making "
-         << "substitution " << target << " to " << full_link_name;
-     return {};
-  }
-
-  if (!FileExists(full_link_name)) {
-     LOG(WARNING) << "Link file " << full_link_name << "missing; not making "
-         << "substitution " << target << " to " << full_link_name;
-     return {};
-  }
-
-  CF_EXPECTF(unlink(full_link_name.c_str()) == 0, "{}", StrError(errno));
-  CF_EXPECT(Symlink(target, full_link_name));
-  return {};
-}
-
 bool SubstituteCheckTargetExists(const fetch::HostPkgMigrationConfig& config,
                                  std::string_view target_keyword) {
   for (int j = 0; j < config.symlinks_size(); j++) {
@@ -165,6 +154,7 @@ Result<void> SubstituteWithMarker(const std::string& target_dir,
             "failed parsing debian_substitution_marker file");
   auto run_cvd_substituted =
       SubstituteCheckTargetExists(config, kRunCvdKeyword);
+  static const std::string common_dir = CF_EXPECT(GetCuttlefishCommonDir());
   for (int j = 0; j < config.symlinks_size(); j++) {
     // TODO(b/452945156): The sensors simulator is launched by run_cvd, so these
     // two components must always be substituted together. Between May 2025 and
@@ -182,7 +172,11 @@ Result<void> SubstituteWithMarker(const std::string& target_dir,
                       "substituted as well.";
       continue;
     }
-    CF_EXPECT(Substitute(target_dir, config.symlinks(j).link_name()));
+
+    std::string link_name = config.symlinks(j).link_name();
+    std::string target = fmt::format("{}/{}", common_dir, link_name);
+    std::string full_link_name = fmt::format("{}/{}", target_dir, link_name);
+    CF_EXPECT(Substitute(target, full_link_name));
   }
   return {};
 }