Separate SystemError from SysError

Most of this is a `catch SysError` -> `catch SystemError` sed. This
is a rather pure-churn change I would like to get out of the way. **The
intersting part is `src/libutil/error.hh`.**

On Unix, we will only throw the `SysError` concrete class, which has
the same constructors that `SystemError` used to have.

On Windows, we will throw `WinError` *and* `SysError`. `WinError`
(which will be created in a later PR), will use a `DWORD` instead of
`int` error value, and `GetLastError()`, which is the Windows equivalent
of the `errno` machinery. Windows will *also* use `SysError` because
Window's "libc" (MSVCRT) implements the POSIX interface, and we use it
too.

As the docs describe, while we *throw* one of the 3 choices above (2
concrete classes or the alias), we should always *catch* `SystemError`.
This ensures no matter how the implementation changes for Windows (e.g.
between `SysError` and `WinError`) the catching logic stays the same
and stays correct.

Co-Authored-By volth <volth@volth.com>
Co-Authored-By Eugene Butler <eugene@eugene4.com>

src/libstore/build/local-derivation-goal.cc

@@ -1495,7 +1495,7 @@ void LocalDerivationGoal::startDaemon()
                     daemon::processConnection(store, from, to,
                         NotTrusted, daemon::Recursive);
                     debug("terminated daemon connection");
-                } catch (SysError &) {
+                } catch (SystemError &) {
                     ignoreException();
                 }
             });
@@ -1707,7 +1707,7 @@ void LocalDerivationGoal::runChild()
         try {
             if (drv->isBuiltin() && drv->builder == "builtin:fetchurl")
                 netrcData = readFile(settings.netrcFile);
-        } catch (SysError &) { }
+        } catch (SystemError &) { }
 
 #if __linux__
         if (useChroot) {

src/libstore/gc.cc

@@ -413,7 +413,7 @@ void LocalStore::findRuntimeRoots(Roots & roots, bool censor)
                     auto env_end = std::sregex_iterator{};
                     for (auto i = std::sregex_iterator{envString.begin(), envString.end(), storePathRegex}; i != env_end; ++i)
                         unchecked[i->str()].emplace(envFile);
-                } catch (SysError & e) {
+                } catch (SystemError & e) {
                     if (errno == ENOENT || errno == EACCES || errno == ESRCH)
                         continue;
                     throw;

src/libstore/globals.cc

@@ -118,7 +118,7 @@ void loadConfFile()
         try {
             std::string contents = readFile(path);
             globalConfig.applyConfig(contents, path);
-        } catch (SysError &) { }
+        } catch (SystemError &) { }
     };
 
     applyConfigFile(settings.nixConfDir + "/nix.conf");

src/libstore/keys.cc

@@ -19,7 +19,7 @@ PublicKeys getDefaultPublicKeys()
         try {
             SecretKey secretKey(readFile(secretKeyFile));
             publicKeys.emplace(secretKey.name, secretKey.toPublicKey());
-        } catch (SysError & e) {
+        } catch (SystemError & e) {
             /* Ignore unreadable key files. That's normal in a
                multi-user installation. */
         }

src/libstore/local-store.cc

@@ -276,7 +276,7 @@ LocalStore::LocalStore(const Params & params)
                 [[gnu::unused]] auto res2 = ftruncate(fd.get(), settings.reservedSize);
             }
         }
-    } catch (SysError & e) { /* don't care about errors */
+    } catch (SystemError & e) { /* don't care about errors */
     }
 
     /* Acquire the big fat lock in shared mode to make sure that no

src/libstore/optimise-store.cc

@@ -242,7 +242,7 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
     /* Atomically replace the old file with the new hard link. */
     try {
         renameFile(tempLink, path);
-    } catch (SysError & e) {
+    } catch (SystemError & e) {
         if (unlink(tempLink.c_str()) == -1)
             printError("unable to unlink '%1%'", tempLink);
         if (errno == EMLINK) {

src/libstore/remote-fs-accessor.cc

@@ -87,13 +87,13 @@ std::pair<ref<SourceAccessor>, CanonPath> RemoteFSAccessor::fetch(const CanonPat
             nars.emplace(storePath.hashPart(), narAccessor);
             return {narAccessor, restPath};
 
-        } catch (SysError &) { }
+        } catch (SystemError &) { }
 
         try {
             auto narAccessor = makeNarAccessor(nix::readFile(cacheFile));
             nars.emplace(storePath.hashPart(), narAccessor);
             return {narAccessor, restPath};
-        } catch (SysError &) { }
+        } catch (SystemError &) { }
     }
 
     StringSink sink;