fix(partition): 🔇 collapse identical COMMPLACE repeats

cpp/monoprop/detail/partition/CpuTopology.cpp

@@ -22,6 +22,7 @@
 #include <mutex>
 #include <print>
 #include <string>
+#include <string_view>
 #include <utility>
 #include <vector>
 
@@ -343,6 +344,17 @@
                        summary.cpu_list);
 }
 
+auto place_line_is_new(std::string_view line) -> bool {
+    static std::mutex mu;
+    static std::string last;
+    const std::lock_guard lock(mu);
+    if (line == last) {
+        return false;
+    }
+    last = line;
+    return true;
+}
+
 /* ── partition_cpusets ─────────────────────────────────────────────────────── */
 
 auto partition_cpusets(size_t n, size_t group_index, size_t group_count, NodeMask mask) -> std::vector<CpuSet> {

cpp/monoprop/detail/partition/CpuTopology.h

@@ -30,6 +30,7 @@
 #include <cstdint>
 #include <optional>
 #include <string>
+#include <string_view>
 #include <vector>
 
 namespace monoprop::detail::partition {
@@ -131,6 +132,13 @@ struct MaskSummary {
                                      const char *verdict,
                                      const MaskSummary &summary) -> std::string;
 
+/*! @brief True the first time @p line is seen, and whenever it differs from the previous call.
+ *  One process has one placement, so a propagator built 437 times reports it 437 times identically;
+ *  the whole line is compared rather than a once-flag set, because a process that builds an `alone`
+ *  propagator before an MPI one has two placements and both are worth saying. Process-wide, locked.
+ */
+[[nodiscard]] auto place_line_is_new(std::string_view line) -> bool;
+
 //! Whether the launcher has already handed this rank a private slice of the node, or the node's CPUs are shared.
 enum class NodeMask { Shared, PerRank };
 

cpp/monoprop/detail/partition/PartitionGroup.h

@@ -23,6 +23,7 @@
 #include <functional>
 #include <memory>
 #include <mutex>
+#include <string>
 #include <thread>
 #include <type_traits>
 #include <vector>
@@ -211,14 +212,15 @@ class PartitionGroup {
         }
         // No summary is "unknown" rather than a plausible zero: some mask did not fit the window.
         const auto sum = summarize_masks(masks, peers, kWords, static_cast<size_t>(node_rank_));
-        std::fputs(format_place_line(mpi::rank(parent_),
-                                     node_rank_,
-                                     node_size_,
-                                     sum ? verdict : "unknown",
-                                     sum.value_or(MaskSummary{}))
-                       .c_str(),
-                   stderr);
-        std::fflush(stderr);
+        const std::string line = format_place_line(mpi::rank(parent_),
+                                                   node_rank_,
+                                                   node_size_,
+                                                   sum ? verdict : "unknown",
+                                                   sum.value_or(MaskSummary{}));
+        if (place_line_is_new(line)) {
+            std::fputs(line.c_str(), stderr);
+            std::fflush(stderr);
+        }
     }
 
     auto make_transport_() -> void {

cpp/tests/cpu_topology_tests.cpp

@@ -438,6 +438,19 @@ BOOST_AUTO_TEST_CASE(cpu_topology_place_line_reports_every_field) {
                       "cpu_list=0-127\n");
 }
 
+// 437 identical lines in one 585-test run is noise; a CHANGED line still has to get through.
+BOOST_AUTO_TEST_CASE(cpu_topology_place_line_repeats_are_collapsed) {
+    // Distinctive, so a real COMMPLACE line emitted by some other case cannot be the previous value.
+    const std::string a = "COMMPLACE test-a\n";
+    const std::string b = "COMMPLACE test-b\n";
+    BOOST_CHECK(partition::place_line_is_new(a));
+    BOOST_CHECK(!partition::place_line_is_new(a));
+    BOOST_CHECK(!partition::place_line_is_new(a));
+    BOOST_CHECK(partition::place_line_is_new(b)); // a different placement is not a repeat
+    BOOST_CHECK(!partition::place_line_is_new(b));
+    BOOST_CHECK(partition::place_line_is_new(a)); // and neither is coming back to the first one
+}
+
 // The state summarize_masks refuses to classify must SAY unknown rather than print a plausible zero.
 BOOST_AUTO_TEST_CASE(cpu_topology_place_line_unknown_is_not_a_verdict) {
     BOOST_CHECK_EQUAL(partition::format_place_line(0, 0, 1, "unknown", partition::MaskSummary{}),