p2p: Avoid InitError when downgrading peers.dat

fixes #24188
When downgrading, a peers.dat with a future version that has a minimum
required version larger than the downgraded version would cause an InitError.

This commit changes this behavior to overwrite the existing peers.dat with
a new empty one, while creating a backup in peers.dat.bak.

3 files changed, 16 insertions, 1 deletions

diff --git a/src/addrdb.cpp b/src/addrdb.cpp
index 94518b88d8..9f3e2e7219 100644
--- a/src/addrdb.cpp
+++ b/src/addrdb.cpp
@@ -196,6 +196,15 @@ std::optional<bilingual_str> LoadAddrman(const std::vector<bool>& asmap, const A
         addrman = std::make_unique<AddrMan>(asmap, /* deterministic */ false, /* consistency_check_ratio */ check_addrman);
         LogPrintf("Creating peers.dat because the file was not found (%s)\n", fs::quoted(fs::PathToString(path_addr)));
         DumpPeerAddresses(args, *addrman);
+    } catch (const InvalidAddrManVersionError&) {
+        if (!RenameOver(path_addr, (fs::path)path_addr + ".bak")) {
+            addrman = nullptr;
+            return strprintf(_("Failed to rename invalid peers.dat file. Please move or delete it and try again."));
+        }
+        // Addrman can be in an inconsistent state after failure, reset it
+        addrman = std::make_unique<AddrMan>(asmap, /* deterministic */ false, /* consistency_check_ratio */ check_addrman);
+        LogPrintf("Creating new peers.dat because the file version was not compatible (%s). Original backed up to peers.dat.bak\n", fs::quoted(fs::PathToString(path_addr)));
+        DumpPeerAddresses(args, *addrman);
     } catch (const std::exception& e) {
         addrman = nullptr;
         return strprintf(_("Invalid or corrupt peers.dat (%s). If you believe this is a bug, please report it to %s. As a workaround, you can move the file (%s) out of the way (rename, move, or delete) to have a new one created on the next start."),
diff --git a/src/addrman.cpp b/src/addrman.cpp
index 3a845b5b6e..f91a979934 100644
--- a/src/addrman.cpp
+++ b/src/addrman.cpp
@@ -248,7 +248,7 @@ void AddrManImpl::Unserialize(Stream& s_)
     s >> compat;
     const uint8_t lowest_compatible = compat - INCOMPATIBILITY_BASE;
     if (lowest_compatible > FILE_FORMAT) {
-        throw std::ios_base::failure(strprintf(
+        throw InvalidAddrManVersionError(strprintf(
             "Unsupported format of addrman database: %u. It is compatible with formats >=%u, "
             "but the maximum supported by this version of %s is %u.",
             uint8_t{format}, uint8_t{lowest_compatible}, PACKAGE_NAME, uint8_t{FILE_FORMAT}));
diff --git a/src/addrman.h b/src/addrman.h
index 0646ef368d..472282833b 100644
--- a/src/addrman.h
+++ b/src/addrman.h
@@ -17,6 +17,12 @@
 #include <utility>
 #include <vector>
 
+class InvalidAddrManVersionError : public std::ios_base::failure
+{
+public:
+    InvalidAddrManVersionError(std::string msg) : std::ios_base::failure(msg) { }
+};
+
 class AddrManImpl;
 
 /** Default for -checkaddrman */