pool: guard the share-target floor where the old cap branch was dead

ShareTargetFromDifficulty ended with

    if (t < diff1 && difficulty < 1.0) t = ~uint256(0);

which can never fire: t = diff1/difficulty, so difficulty < 1.0 implies
t > diff1 and the two conditions exclude each other. The branch was
reaching for a real hazard, though, so say the hazard directly instead
of deleting the guard: diff1 is ~uint256(0) >> 12, a hair under 2^244,
so the true share target stops fitting in a uint256 once difficulty
reaches 2^-12 (1/4096), and the fixed-point divisor
(uint64)(difficulty * 65536.0) truncates to zero -- a division by zero
-- for any difficulty under 2^-16. Past the first threshold
getuint256() would return the low 256 bits of an oversized quotient, a
wrapped target that is arbitrarily HARDER than difficulty 1. Both
thresholds sit far below VARDIFF_MIN (0.001), so nothing live comes
near this; the guard exists for any caller that bypasses vardiff. At
those difficulties every hash is meant to be a share, so return
all-ones, the easiest representable target. Boundary verified
numerically against the exact fixed-point algorithm: at difficulty
exactly 1/4096 the quotient is 2^256 - 4096 (fits), one ulp below it
overflows, and below 2^-16 the divisor is zero.

Closes #11

1 files changed, 17 insertions, 4 deletions

diff --git a/src/rpc.cpp b/src/rpc.cpp
index a4b53ca..82f895b 100644
--- a/src/rpc.cpp
+++ b/src/rpc.cpp
@@ -115,15 +115,28 @@ static uint256 ShareTargetFromDifficulty(double difficulty)
 {
     if (difficulty <= 0.0) difficulty = 1.0;
     static const uint256 diff1 = CBigNum(bnProofOfWorkLimit).getuint256();
+    // Representability floor. shareTarget = diff1 / difficulty, and diff1 is
+    // ~uint256(0) >> 12, i.e. a hair under 2^244 -- so the true target stops
+    // fitting in a uint256 once difficulty reaches 2^-12 (= 1/4096), and the
+    // fixed-point divide below breaks even earlier in a second way: the
+    // (uint64)(difficulty * 65536.0) divisor truncates to zero for any
+    // difficulty under 2^-16. Both thresholds sit far below VARDIFF_MIN
+    // (0.001), so live operation never comes near this, but a caller that
+    // bypasses vardiff (or a future VARDIFF_MIN) must not divide by zero or
+    // hand back getuint256()'s low-256-bit truncation of an oversized
+    // quotient -- a wrapped target is not merely wrong, it is arbitrarily
+    // HARDER than difficulty 1. At these difficulties every hash is meant to
+    // be a share, so the honest answer is the easiest representable target.
+    // (This replaces a dead branch that read `if (t < diff1 && difficulty <
+    // 1.0)`: t = diff1/difficulty, so difficulty < 1.0 implies t > diff1 and
+    // the two conditions could never both hold.)
+    if (difficulty < 1.0 / 4096.0) return ~uint256(0);
     // shareTarget = diff1 / difficulty
     CBigNum bn; bn.setuint256(diff1);
     // Multiply by 2^16 to keep precision, divide, shift back
     bn <<= 16;
     bn /= (uint64)(difficulty * 65536.0);
-    uint256 t = bn.getuint256();
-    // Cap at all-ones (maximum, easiest possible)
-    if (t < diff1 && difficulty < 1.0) t = ~uint256(0);
-    return t;
+    return bn.getuint256();
 }
 
 // Expected number of hashes to find one share at a given Stratum difficulty,